Online Etymology Dictionary

clambake (n.): 1835, from clam (n.) + bake (n.).
clamber (v.): late 14c., possibly frequentative of Middle English climben "to climb" (preterit clamb), or akin to Old Norse klembra "to hook (oneself) on." Related: Clambered; clambering.
clammy (adj.): late 14c., probably from Middle English clam "viscous, sticky, muddy" (mid-14c.), from Old English clæm "mud, sticky clay," from Proto-Germanic *klaimaz "clay" (cf. Flemish klammig, Low German klamig "sticky, damp," Old English clæman "to smear, plaster;" cf. clay).
clamor (n.): late 14c., from Old French clamour (12c., Modern French clameur), from Latin clamor "a shout," from clamare "to cry out" (see claim (v.)).
clamor (v.): late 14c., from clamor (n.). Related: Clamored; clamoring.
clamorous (adj.): c.1400, from Middle French clamoreux or directly from Medieval Latin clamorosus, from Latin clamor (see clamor (n.)). Related: Clamorously; clamorousness.
clamour: chiefly British English spelling of clamor (q.v.); for spelling, see -or. Related: Clamouring; clamourous.
clamp (n.): device for fastening, c.1300, probably from clamb, originally past tense of climb, or from Middle Dutch clampe (Dutch klamp), from West Germanic *klamp- "clamp, cleat" (cf. Middle Low German klampe "clasp, hook," Old High German klampfer "clip, clamp," Old English clamm "fetter;" see clam (n.)).
clamp (v.): 1670s, from clamp (n.). Related: Clamped; clamping.
clan (n.): early 15c., from Gaelic clann "family, stock, offspring," akin to Old Irish cland "offspring, tribe," both from Latin planta "offshoot" (see plant (n.)). The Goidelic branch of Celtic (including Gaelic) had no initial p-, so it substituted k- or c- for Latin p-. The same Latin word in (non-Goidelic) Middle Welsh became plant "children."
clandestine (adj.): 1560s, from Latin clandestinus "secret, hidden," from clam "secretly," from adverbial derivative of base of celare "to hide" (see cell), perhaps on model of intestinus "internal." Related: Clandestinely.
clang (v.): 1570s, echoic (originally of trumpets and birds), akin to or from Latin clangere "resound, ring," and Greek klange "sharp sound," from PIE *klang-, nasalized form of root *kleg- "to cry, sound." Related: Clanged; clanging. The noun is attested from 1590s.
clangor (n.): 1590s, from Latin clangor "sound of trumpets (Virgil), birds (Ovid), etc.," from clangere "to clang," echoic (cf. clang). Related: Clangorous.
clank: 1610s (v.), 1650s (n.), perhaps echoic, perhaps a blend of clang and clink, perhaps from a Low German source (cf. Middle Dutch clank, Middle Low German klank).
clannish (adj.): 1769, from clan + -ish. Related: Clannishly; clannishness.
clap (v.): Old English clæppan "to clap, throb, beat," echoic (cf. Old Frisian klapa "to beat," Old High German klaphon, Old Saxon klapunga). Of hands, to beat them together to get attention or express joy, from late 14c. Related: Clapped; clapping.
clap (n.1): "loud noise," c.1200, from clap (v.). Of thunder, late 14c.
clap (n.2): "gonorrhea," 1580s, of unknown origin, perhaps from Middle English claper, from Old French clapoire, originally "rabbit burrow" but given a slang extension to "brothel" and also the name of a disease of some sort. In English originally also a verb, "to infect with clap."
clapboard (n.): 1520s, partial translation of Middle Dutch klapholt (borrowed into English late 14c.), from klappen "to fit" + Low German holt "wood, board" (see holt).
clapper (n.): late 13c., agent noun from clap (v.). Meaning "tongue of a bell" is from late 14c.
claptrap (n.): c.1730, "trick to 'catch' applause," a stage term; from clap (v.) + trap (n.). Extended sense of "cheap, showy language" is from 1819; hence "nonsense, rubbish."
claque (n.): 1864, from French claquer "to clap" (16c.), echoic (cf. clap (v.)). Modern sense of "political followers" is transferred from that of organized applause at theater.
Clara: fem. personal name, from Latin Clara, from fem. of clarus "bright, shining, clear" (see clear (adj.) and cf. Claire). Derivatives include Clarisse, Clarabel. The native form Clare was common in medieval England, perhaps owing to the popularity of St. Clare of Assisi.
Clarence: surname, from Medieval Latin Clarencia, name of dukedom created 1362 for Lionel, third son of Edward III, so called from town of Clare, Suffolk, whose heiress Lionel married. Used as a masc. proper name from late 19c. As a type of four-wheeled closed carriage, named for the Duke of Clarence, later William IV.
clarendon (n.): type face, 1845, evidently named for the Clarendon press at Oxford University, which was set up 1713 in the Clarendon Building, named for university Chancellor Edward Hyde, 1st Earl of Clarendon.
claret (n.): mid-15c., "light-colored wine," from Old French (vin) claret "clear (wine), light-colored red wine" (also "sweetened wine," a sense in English from late 14c.), from Latin clarus "clear" (see clear (adj.)). Narrowed meaning "red wine of Bordeaux" first attested 1700.
clarification (n.): 1610s, from French clarification, from Late Latin clarificationem (nominative clarificatio), noun of action from past participle stem of clarificare (see clarify). The meaning "statement in a publication revising or expanding an earlier published statement in it but stopping short of a correction" is attested by 1969.
clarify (v.): early 14c., from Old French clarifiier (12c.), from Late Latin clarificare "to make clear," also "to glorify," from Latin clarificus "brilliant," from clarus "clear, distinct" (see clear (adj.)) + root of facere "to make, do" (see factitious). Related: Clarified; clarifying.
clarinet (n.): 1768, from French clarinette (18c.), diminutive of clarine "little bell" (16c.), from fem. of adjective clarin, from clair, cler (see clear (adj.)). The instrument said to have been invented c.1700 by J.C. Denner of Nuremberg, Germany. Alternative form clarionet is attested from 1784.
After the hautboy came the clarinet. This instrument astonished every beholder, not so much, perhaps, on account of its sound, as its machinery. One that could manage the keys of a clarinet, forty five years ago, so as to play a tune, was one of the wonders of the age. Children of all ages would crowd around the performer, and wonder and admire when the keys were moved. [Nathaniel D. Gould, "Church Music in America," Boston, 1853]
clarion (n.): early 14c., from Old French clarion "(high-pitched) trumpet, bugle," from Medieval Latin clarionem (nominative clario) "a trumpet," from Latin clarus "clear" (see clear (adj.)). Clarion call is attested from 1838.
clarity (n.): c.1300, from Old French clarté "clarity, brightness," from Latin claritas "clearness, brightness, splendor," from clarare "make clear," from clarus "clear" (see clear (adj.)).
Clark: surname, from common Middle English alternative spelling of clerk (n.). In many early cases it is used of men who had taken minor orders.
clash: c.1500 (v.), 1510s (n.), of imitative origin; the figurative sense in reference to strife or battle is first attested 1620s. Of colors, "to go badly together," first recorded 1894. Related: Clashed; clashing.
clasp (n.): c.1300, claspe, perhaps metathesis alteration of clapse, and thus from Old English clyppan "clasp" (see clip (v.2)).
clasp (v.): late 14c., from clasp (n.). Related: Clasped; clasping.
class (n.): c.1600, "group of students," from French classe (14c.), from Latin classis "class, division, army, fleet," especially any one of the six orders into which Servius Tullius divided the Roman people for the purpose of taxation; traditionally originally "the people of Rome under arms," and thus akin to calare "to call (to arms)," from PIE root *kele- (2) "to shout" (see claim (v.)).

School and university sense of "course, lecture" (1650s) is from the notion of a form or lecture reserved to scholars who had attained a certain level. Natural history sense is from 1753. Meaning "a division of society according to status" (upper, lower, etc.) is from 1772. Class-consciousness (1903) is from German klassenbewusst.
class (v.): 1705, "to divide into classes," from class (n.). Sense of "to place into a class" is from 1776. Related: Classed; classing.
classic (adj.): 1610s, from French classique (17c.), from Latin classicus "relating to the (highest) classes of the Roman people," hence, "superior," from classis (see class). Originally in English "of the first class;" meaning "belonging to standard authors of Greek and Roman antiquity" is attested from 1620s. Classics is 1711, and is the earliest form of the word to be used as a noun.
classical (adj.): 1590s, "of the highest rank," from classic + -al (1). Classical music (1836) was defined originally against romantic music.
[I]n general, as now used, the term classical includes the composers active in instrumental music from somewhere about 1700 to say 1830. Hence the list includes among the great names those of Bach, his sons, Haydn, Mozart, Beethoven, Clementi, Dussek, Pleyel, Cramer, etc. The next step beyond the term classical is "modern romantic," the composers of which school may be taken to include all the writers for pianoforte from about 1829 (when Mendelssohn published the first "Songs without Words") down to the present. The term romantic in this sense means strongly marked, extraordinary, intending to tell stories and the like. ["Music, Its Ideals and Methods," W.S.B. Mathews, 1897]
But already by 1880s it was acknowledged the term had a double sense: Music that had withstood the test of time, as well as music of a style contrasted to "romantic." Later (early 20c.) it was contrasted to jazz (in this sense more often with reference to the orchestras than to the music itself). Still later in contrast to popular music generally.
classicism (n.): "classical style in art or literature," 1837, from classic + -ism.
classification (n.): 1790, from classify + -ation, or from French classification.
classifieds (n.): "newspaper advertisements arranged by classes," 1913, short for classified advertisements (see classify).
classify (v.): 1799, from French classifier, from classe (see class (n.)) + -fier (see -fy). Related: Classified; classifying.
classism (n.): "distinction of class," 1842, from class (n.) + -ism.
classmate (n.): 1713, from class (n.) + mate (n.).
classroom (n.): 1870, from class (n.) + room (n.).
classy (adj.): 1891, from class (n.) + -y (2). Related: Classily; classiness.
clastic (adj.): 1875, from Greek klastos "broken in pieces," from klan, klaein "to break," from PIE *kla-, variant of root *kel- "to strike."
clatter (v.): late Old English clatrung, probably from Old English *clatrian, of imitative origin. Cf. Middle Dutch klateren, East Frisian klatern, dialectal German klattern. The noun is attested from mid-14c.
Clatterer or clatterfart, which wyl disclose anye light secreate. [Richard Huloet, "Abecedarium Anglo-Latinum," 1552.]
Claudia: fem. proper name, from Latin Claudius (m.), from the Roman gens Claudia, an old and respected family in the city, perhaps ultimately from claudus "lame."