Etymology
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adage (n.)

"brief, familiar proverb," 1540s, French adage (16c.), from Latin adagium "adage, proverb," apparently a collateral form of adagio, from ad "to" (see ad-) + *agi-, root of aio "I say," which is perhaps cognate with Armenian ar-ac "proverb," asem "to say." But some find this unlikely and suggest the second element might be related to agein "set in motion, drive, urge" (from PIE root *ag- "to drive, draw out or forth, move"). Related: Adagial.

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tartar (n.)
"bitartrate of potash" (a deposit left during fermentation), late 14c., from Old French tartre, from Medieval Latin tartarum, from late Greek tartaron "tartar encrusting the sides of wine casks," perhaps of Semitic origin, but if so the exact source has not been identified; Arabic is unlikely because of the early date of the word in Latin. The purified substance is cream of tartar. Used generally in 17c. of encrustations from liquid contact; specific meaning "encrustation on teeth" (calcium phosphate) is first recorded 1806.
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radius (n.)

1590s, "cross-shaft, straight rod or bar," from Latin radius "staff, stake, rod; spoke of a wheel; ray of light, beam of light; radius of a circle," a word of unknown origin. Perhaps related to radix "root," but de Vaan finds that "unlikely." The classical plural is radii

The geometric sense of "straight line drawn from the center of a circle to the circumference" is recorded from 1650s. Meaning "circular area of defined distance around some place" is attested from 1853. As the name of the shorter of the two bones of the forearm from 1610s in English (the Latin word had been used thus by the Romans).

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skull (n.)
"bony framework of the head," c. 1200, probably from Old Norse skalli "a bald head, skull," a general Scandinavian word (compare Swedish skulle, Norwegian skult), probably related to Old English scealu "husk" (see shell (n.)). But early prominence in southwestern texts suggests rather origin from a Dutch or Low German cognate (such as Dutch schol "turf, piece of ice," but the sense of "head bone framework" is wanting). Derivation from Old French escuelle seems unlikely on grounds of sound and sense. Old English words for skull include heafod-bolla.
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testimony (n.)
c. 1400, "proof or demonstration of some fact, evidence, piece of evidence;" early 15c., "legal testimony, sworn statement of a witness," from Old North French testimonie (Old French testimoine 11c.), from Latin testimonium "evidence, proof, witness, attestation," from testis "a witness, one who attests" (see testament) + -monium, suffix signifying action, state, condition. Despite the common modern assertion, the sense of the word is unlikely to have anything to do with testicles (see testis).

Earliest attested sense in English is "the Ten Commandments" (late 14c.), from Vulgate use of Late Latin testimonium, along with Greek to martyrion (Septuagint), translations of Hebrew 'eduth "attestation, testimony" (of the Decalogue), from 'ed "witness."
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spar (v.)

late 14c., "go quickly, rush, dart, spring;" c. 1400, "to strike or thrust," perhaps from French esparer "to kick" (Modern French éparer), from Italian sparare "to fling," from Latin ex- (see ex-) + parare "make ready, prepare," hence "ward off, parry" (from PIE root *pere- (1) "to produce, procure"). Etymologists consider a connection with spur unlikely. Used in 17c. in reference to preliminary actions in a cock fight; figurative sense of "to dispute, bandy with words" is from 1690s. Extension to humans, in a literal sense, with meaning "to engage in or practice boxing" is attested from 1755. Related: Sparred; sparring.

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shackle (n.)

kind of fetter, especially for the wrist or ankle of a prisoner, Middle English shakel, from Old English sceacel, sceacul "shackle, fetter," probably also in a general sense "a link or ring of a chain," from Proto-Germanic *skakula- (source also of Middle Dutch, Dutch schakel "link of a chain, ring of a net," Old Norse skökull "pole of a carriage"), of uncertain origin. According to OED, the common notion of "something to fasten or attach" makes a connection with shake unlikely. Figurative sense of "anything which hinders or restrains" is by early 13c. Related: Shackledom "marriage" (1771); shacklebone "the wrist" (1570s) is Scottish or northern dialect.

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saunter (v.)

c. 1500, santren "to muse, be in reverie," a word of uncertain origin. The meaning "walk with a leisurely gait" is from 1660s, and may be a different word which, despite many absurd speculations, also is of unknown origin. Klein prints the theory (held by Skeat and Murray) that this sense of the word derives via Anglo-French sauntrer (mid-14c.) from French s'aventurer "to take risks." Century Dictionary finds the theory involves difficulties but "it is the only one that has any plausibility," but OED finds it "unlikely." Also see here. Related: Sauntered; saunterer; sauntering.

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niggard (n.)

"mean or stingy person, miser," late 14c., nigard, nygard, nygart, also with a variant nigoun, nygun (c. 1300), a word of uncertain origin. The suffix suggests French origin (see -ard), but the root word is possibly from earlier nig "stingy" (c. 1300), which is perhaps from a Scandinavian source related to Old Norse *hniggw, related to hnøggr "stingy," from Proto-Germanic *khnauwjaz (source of Swedish njugg "close, careful," German genau "precise, exact"). Perhaps also related to Old English hneaw "stingy, niggardly," which did not survive in Middle English. A noun nig "niggardly person" is attested from c. 1300, but OED considers this unlikely to be the source of the longer word.

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explore (v.)

1580s, "to investigate, examine," a back-formation from exploration, or else from French explorer (16c.), from Latin explorare "investigate, search out, examine, explore," said to be originally a hunters' term meaning "set up a loud cry," from ex "out" (see ex-) + plorare "to weep, cry." Compare deplore. De Vaan notes modern sources that consider "the ancient explanation, ... that the verb explorare originally meant 'to scout the hunting area for game by means of shouting'" to be "not unlikely." Second element also is explained as "to make to flow," from pluere "to flow." Meaning "to go to a country or place in quest of discoveries" is first attested 1610s. Related: Explored; exploring.

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