acetylene (n.)
gaseous hydrocarbon, by 1862, from French acétylène, coined 1860 by French chemist Pierre Eugène Marcellin Berthelot (1827-1907) from chemical ending -ene + acetyl, which was coined from acetic + -yl in 1839 by German chemist Justus von Liebig. Liebig's coinage was in reference to a different radical; acetyl was transferred to its current sense in 1850s, but Berthelot's coinage was based on the original use of acetyl.
ach (interj.)
aspirated form of ah; in English often used to represent German or Celtic speech.
in Homeric language, "a Greek," generally; later restricted to natives or inhabitants of Achaea, a region in the Peloponnesus. The Achaean League after c. 280 B.C.E. was a model for later federal republics. In Latin, Achaicus meant "a Greek."
armor-bearer and faithful friend of Aeneas in the "Aeneid;" The phrase fidus Achates was proverbial for "faithful friend, loyal and devoted companion." The name is from Greek akhates "agate" (see agate).
ache (v.)
Old English acan "suffer continued pain," from Proto-Germanic *akanan, perhaps from a PIE root *ag-es- "fault, guilt," represented also in Sanskrit and Greek, which is perhaps imitative of groaning.

Originally the verb was pronounced "ake," the noun "ache" (as in speak/speech). The noun changed pronunciation to conform to the verb, but the spelling of both was changed to ache c. 1700 on a false assumption of a Greek origin (specifically Greek akhos "pain, distress," which rather is a distant relation of awe (n.)). Related: Ached; aching.
ache (n.)
"continuing pain," early 15c., æche, ece "an ache, pain," from Old English æce, from Proto-Germanic *akiz, from same source as ache (v.), which see for the unusual evolution of spelling and pronunciation.
1580s, fabled river of the Lower World in Greek mythology, from Greek Akheron, name of several real rivers, also the mythical river of the Underworld. The name perhaps means "forming lakes" (compare Greek akherousai "marsh-like water"), from PIE root *eghero- "lake" (source of Lithuanian ežeras, ažeras, Old Prussian assaran, Old Church Slavonic jezero "lake"). The derivation from Greek akhos "woe" is considered folk etymology. The name was later given to rivers in Greece and Italy that flowed through dismal surroundings or disappeared underground. Related: Acherontic.
achievable (adj.)
1620s; see achieve (v.) + -able. Related: Achievably; achievability.
achieve (v.)
early 14c., "to perform, execute, accomplish;" late 14c., "gain as a result of effort," from Old French achever (12c.) "to finish, accomplish, complete," from phrase à chef (venir) "at an end, finished," or Vulgar Latin *accapare, from Late Latin ad caput (venire); both the French and Late Latin phrases meaning literally "to come to a head," from ad "to" (see ad-) + stem of Latin caput "head" (from PIE root *kaput- "head").
The Lat. caput, towards the end of the Empire, and in Merov[ingian] times, took the sense of an end, whence the phrase ad caput venire, in the sense of to come to an end .... Venire ad caput naturally produced the Fr. phrase venir à chef = venir à bout. ... From this chief, O.Fr. form of chef (q.v.) in sense of term, end, comes the Fr. compd. achever = venir à chef, to end, finish. [Auguste Brachet, "An Etymological Dictionary of the French Language," transl. G.W. Kitchin, Oxford, 1878]
Related: Achieved; achieving.
achievement (n.)
late 15c., "act of completing" (something), from Middle French achèvement "a finishing," noun of action from Old French achever "to finish, accomplish" (see achieve). Meaning "thing achieved" is recorded from 1590s.
Greek hero of the Trojan War stories, bravest, swiftest, and handsomest of Agamemnon's army before Troy, he was son of Thetis and Peleus. His name is perhaps a compound of akhos "pain, grief" (see awe) + laos "the people, a people" (see lay (adj.)); or else it is from Pre-Greek (non-IE). Related: Achillean.
Achilles tendon (n.)
from Modern Latin tendo Achillis, first used by German surgeon Lorenz Heister (1683-1758) and so-called in reference to the one vulnerable spot of the Greek hero Achilles, whose mother held him by the heel when she dipped him in the River Styx to render him invulnerable (this story is not in Homer and not found before 1c. C.E.). Earlier Achilles' sinew, from Modern Latin chorda Achillis, coined 1693 by Dutch anatomist Philip Verheyden when dissecting his own amputated leg. Hence figurative use of heel of Achillies for "vulnerable spot" (1810).
achromatic (adj.)
1766, from a- (3) "not, without" + chromatic. Related: Achromatically.
achtung (interj.)
German word used to command attention, from German achtung, from acht (n.) "attention, care, heed, consideration," achten (v.) "pay attention to, regard, esteem, respect," from Old High German ahton "pay attention to," a general Germanic word akin to Old English eahtian "to estimate, esteem, consider, praise," but with no living native descendants in English.
achy (adj.)
1875, first recorded in George Eliot's letters, from ache (n.) + -y (2). Middle English had akeful "painful" (early 15c.). Related: Achily; achiness.
acicular (adj.)
"resembling or in the form of small needles," 1794, from Latin acicula "needle, small pin," diminutive of acus "pin," from PIE root *ak- "be sharp, rise (out) to a point, pierce."
acid (n.)
1690s, from acid (adj.); originally loosely applied to any substance tasting like vinegar, in modern chemistry gradually given more precise definitions from early 18c. Slang meaning "LSD-25" first recorded 1966 (see LSD).
When I was on acid I would see things that looked like beams of light, and I would hear things that sounded an awful lot like car horns. [Mitch Hedberg, 1968-2005, U.S. stand-up comic]
Acid rock (type performed or received by people using LSD) is also from 1966; acid house dance music style is 1988, probably from acid in the hallucinogenic sense + house "dance club DJ music style."
acid (adj.)
1620s, "of the taste of vinegar," from French acide (16c.) or directly from Latin acidus "sour, sharp, tart" (also figurative, "disagreeable," etc.), adjective of state from acere "to be sour, be sharp," from PIE root *ak- "be sharp, rise (out) to a point, pierce."

Figurative use in English is from 1775; applied to intense colors from 1916; an acid dye (1888) involves an acid bath. Acid test is American English, 1881, a quick way to distinguish gold from similar metals by application of nitric acid. Acid rain "highly acidity in rain caused by atmospheric pollution" is first recorded 1859 in reference to England. Acid drop as a kind of hard sugar candy flavored with tartaric acid is by 1835, with drop (n.) in the "lozenge" sense.
acidic (adj.)
1877, originally in geology; see acid (n.) + -ic.
acidify (v.)
1784 (implied in acidifying); see acid (adj.) + -ify. Related: Acidified.
acidity (n.)
1610s, from French acidité (16c.) or directly from Latin aciditatem (nominative aciditas) "sourness," noun of quality from Latin acidus "sour, tart" (see acid (adj.)).
acidophilus (adj.)
1920, used of milk fermented by acidophilic bacteria, from acidophil (1900), indicating "easily stained by acid dyes," a hybrid word, from Latin acidus "acidic, sour, tart" (see acid (adj.)) + Greek philos "loving" (see -phile); the bacteria so called because they stain easily with an acid dye.
acidulate (v.)
"make somewhat sour, flavor with an acid," 1704 (implied in acidulated), from Latin acidulus "slightly sour" (see acidulous) + -ate (2). Related: Acidulating; acidulent.
acidulous (adj.)
"sub-acidic, slightly sour" (of cream of tartar, oranges, etc.), 1766, also used figuratively for "sour-tempered;" from Latin acidulus "slightly sour," a diminutive of acidus (see acid (adj.)).
British oral code for letter -a- in wireless and telephone communication, 1898; hence ack-ack "anti-aircraft" (gun, fire, etc.). Compare toc (-t-), emma (-m-).
1939, representing A.A., the military abbreviation for anti-aircraft (see ack).
acknowledge (v.)
late 15c., "admit or show one's knowledge," a blend of Middle English aknow "admit or show one's knowledge" (from Old English oncnawan "understand, come to recognize," from on (see on (prep.)) + cnawan "recognize;" see know) and Middle English knowlechen "admit, acknowledge" (c. 1200; see knowledge). "By 16th c. the earlier vbs. knowledge and a(c)know ... were obs., and acknowledge took their place" [OED].

In the merger, an unetymological -c- slipped in; perhaps the explanation is that when English kn- became a simple "n" sound, the -c- stepped up to preserve, in this word, the ancient "kn-" sound. Related: Acknowledged; acknowledging.
acknowledgement (n.)
1590s, "act of acknowledging," from acknowledge + -ment. "An early instance of -ment added to an orig. Eng. vb." [OED]. Meaning "token of due recognition" is recorded from 1610s.
acknowledgment (n.)
alternative spelling of acknowledgement. OED deems it "a spelling more in accordance with Eng[lish] values of letters." Compare judgment.
also A.C.L.U., abbreviation of American Civil Liberties Union.
acme (n.)
"highest point," 1560s, from Greek akme "(highest) point, edge; peak of anything," hence "prime (of life, etc.), the best time," from PIE *ak-ma-, suffixed form of root *ak- "be sharp, rise (out) to a point, pierce." In English it was written in Greek letters until c. 1620. The U.S. grocery store chain was founded 1891 in Philadelphia, Pennsylvania.
acne (n.)
skin eruption common during puberty, 1813, from Modern Latin, from aknas, a 6c. Latin clerical misreading of Greek akmas, accusative plural of akme "point" (see acme), from PIE root *ak- "be sharp, rise (out) to a point, pierce." The "pointed" pimples are the source of the medical use.
acold (adj.)
early 14c.; see a- (1), here perhaps intensive + cold (adj.). Or it might be literally "a-cooled," from the past participle of the verb acool "to take cold" (late Old English); "to make cold" (15c.).
acolyte (n.)
early 14c., "inferior officer in the church," from Old French acolite or directly from Medieval Latin acolytus (Late Latin acoluthus), from Greek akolouthos "following, attending on," as a noun, "a follower, attendant," literally "having one way," from a- "together with," copulative prefix (see a- (3)), + keleuthose "a way, road, path, track," from PIE *qeleu- (source also of Lithuanian kelias "way"). The word was in late Old English as acolitus, a Latin form; in early modern English a corrected form acolythe was used.
acomia (n.)
"baldness," Modern Latin, from Greek akomos "hairless, bald," from a- "not, without" (see a- (3)) + kome "hair" (see comet) + abstract noun suffix -ia.
aconite (n.)
poisonous plant (also known as monkshood and wolfsbane), 1570s, from French aconit (16c.), from Latin aconitum, from Greek akoniton, which is of unknown origin. The highly poisonous alkaloid in it, once isolated, was named aconitine (1847).
acorn (n.)
Old English æcern "nut, mast of trees, acorn," a common Germanic word (cognates: Old Norse akarn, Dutch aker, Low German ecker "acorn," German Ecker, Gothic akran "fruit"), originally the mast of any forest tree. It is by most sources said to be related (via notion of "fruit of the open or unenclosed land") to the source of Old English æcer "open land," Gothic akrs "field," Old French aigrun "fruits and vegetables" (from Frankish or some other Germanic source); see acre.

The sense was gradually restricted in Low German, Scandinavian, and English to the most important of the forest produce for feeding swine: the mast of the oak tree. The regular modern form would be *akern; the current spelling emerged 15c.-16c. by folk etymology association with oak (Old English ac) and corn (n.1), neither of which has anything to do with it. Acorn squash is attested by 1937.
acoustic (adj.)
c. 1600, "pertaining to hearing or sound," from French acoustique, from Latinized form of Greek akoustikos "pertaining to hearing," from akoustos "heard, audible," verbal adjective from akouein "to hear," probably from copulative prefix a- (see a- (3)) + koein "to mark, perceive, hear," from PIE *kous- "to hear" (also source of English hear), which is perhaps from root *(s)keu- "to notice, observe" (see caveat).

In reference to material meant to deaden sound, 1924. Of sound reproduced mechanically (rather than electrically) from 1932 in reference to gramophone players; acoustic guitar (as opposed to electric) attested by 1958. Related: Acoustical; acoustically.
acoustics (n.)
1680s, "the science of sound," from acoustic (also see -ics). Meaning "acoustic properties" of a building, etc., attested from 1885.
acquaint (v.)
early 13c., "make oneself known" (reflexive, now obsolete); early 14c., "to gain for oneself personal knowledge of," from Old French acointer "make known; make or seek acquaintance of," from Vulgar Latin accognitare "to make known," from Latin accognitus "acquainted with," past participle of accognoscere "know well," from ad "to" (see ad-) + cognitus, past participle of cognoscere "come to know" (see cognizance).

Meaning "to inform (someone of something), furnish with knowledge or information" is from 1550s. Related: Acquainted; acquainting.
acquaintance (n.)
c. 1300, "state of being acquainted;" late 14c., "person with whom one is acquainted;" also "personal knowledge;" from Old French acointance "acquaintance, friendship, familiarity," noun of action from acointer "make known" (see acquaint). Acquaintant (17c.), would have been better in the "person known" sense but is now obsolete. Fowler regards acquaintanceship (1792) as a "needless variant."
acquainted (adj.)
early 13c., "personally known;" past participle adjective from acquaint (v.). Of skills, situations, etc., from late 15c.
I have been one acquainted with the night.
I have walked out in rain — and back in rain.
I have outwalked the furthest city light.

[Robert Frost, from "Acquainted with the Night"]
Acquaint also was used as an adjective (late 13c.) "acquainted."
acquiesce (v.)
1610s, "remain at rest" (a sense now obsolete); 1650s as "agree tacitly, concur," from Middle French acquiescer "to yield or agree to; be at rest," (14c.), from Latin acquiescere/adquiescere "become quiet, remain at rest, rest, repose," thus "be satisfied with, be content," from ad "to" (see ad-) + quiescere "become quiet," from quies (genitive quietis) "rest, quiet" (see quiet (n.)). Related: Acquiesced; acquiescing.
acquiescence (n.)
1630s, "rest, quiet, satisfaction," from Middle French acquiescence, noun of action from acquiescer (see acquiesce). Meaning "silent consent, passive assent" is recorded from 1640s.
acquiescent (adj.)
"disposed to yield, submissive," 1690s (implied in acquiescently), from Latin acquiescentem (nominative acquiescens), present participle of acquiescere "become quiet, remain at rest" (see acquiesce).
acquire (v.)
"to get or gain, obtain," mid-15c., acqueren, from Old French aquerre "acquire, gain, earn, procure" (12c., Modern French acquérir), from Vulgar Latin *acquaerere, corresponding to Latin acquirere/adquirere "to get in addition to, accumulate, gain," from ad "to," here perhaps emphatic (see ad-), + quaerere "to seek to obtain" (see query (v.)). Reborrowed in current form from Latin c. 1600. Related: Acquired; acquiring.
acquired (adj.)
c. 1600, "gained by effort," past participle adjective from acquire. Of diseases, "occurring after birth, thus not dependent on heredity," 1842 (opposed to congenital); acquired immune deficiency is attested by 1980; acquired immune deficiency syndrome by 1982. Acquired taste is attested from 1734.
acquirement (n.)
1620s, from acquire + -ment. Perhaps modeled on French acquerement (16c.).
acquisition (n.)
late 14c., "act of obtaining," from Old French acquisicion "purchase, acquirement" (13c., Modern French acquisition) or directly from Latin acquisitionem (nominative acquisitio), noun of action from past participle stem of acquirere "get in addition, accumulate," from ad "to," here perhaps emphatic (see ad-), + quaerere "to seek to obtain" (see query (v.)). Meaning "thing obtained" is from late 15c. The vowel change of -ae- to -i- in Latin is due to a phonetic rule in that language involving unaccented syllables in compounds.
acquisitive (adj.)
1630s, "owned through acquisition" (now obsolete, this sense going with acquired), from Latin acquisit-, past participle stem of acquirere "accumulate, gain" (see acquire) + -ive. Meaning "given to acquisition, avaricious" is from 1826 (implied in acquisitiveness). Related: Acquisitively (1590s).