Online Etymology Dictionary

clairvoyant (adj.): "having psychic gifts," 1837, earlier "having insight" (1670s), from special use of French clairvoyant "clear-sighted, discerning, judicious" (13c.), from clair (see clear (adj.)) + voyant "seeing," present participle of voir, from Latin videre "to see" (see vision).
clam (n.): bivalve mollusk, c.1500, in clam-shell, originally Scottish, apparently a particular use from Middle English clam "pincers, vice, clamp" (late 14c.), from Old English clamm "bond, fetter, grip, grasp," from Proto-Germanic *klam- "to press or squeeze together" (cf. Old High German klamma "cramp, fetter, constriction," German Klamm "a constriction"). If this is right then the original reference is to the shell. Clam-chowder attested from 1822. To be happy as a clam is from 1833, but the earliest uses do not elaborate on the notion behind it, unless it be self-containment.
clam (v.): "to dig for clams," 1630s, American English, from clam (n.). Clam up "be quiet" is 1916, American English, but clam was used in this sense as an interjection mid-14c.
clam-shell (n.): c.1500; see clam (n.) + shell (n.). As "hinged iron box or bucket used in dredging," from 1877.
clambake (n.): 1835, American English, from clam (n.) + bake (n.). By 1937 in jazz slang transferred to "an enjoyable time generally," especially "jam session."
clamber (v.): "to climb with difficulty using hands and feet," 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.
clamjamphry (n.): contemptuous word for "a collection of persons, mob," 1816, of unknown origin; first in Scott, so perhaps there's a suggestion of clan in it.
clammy (adj.): "soft and sticky," 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). With -y (2). Related: Clammily; clamminess.
clamor (n.): late 14c., from Old French clamor "call, cry, appeal, outcry" (12c., Modern French clameur), from Latin clamor "a shout, a loud call" (either friendly or hostile), 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 "a shout" (see clamor (n.)). Related: Clamorously; clamorousness.
clamour: chiefly British English spelling of clamor (q.v.); for spelling, see -or. Related: Clamoured; clamouring; clamourous.
clamp (n.): device for fastening, c.1300, probably from clamb, perhaps originally past tense of climb (v.), 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;" also probably related to Middle Dutch klamme "a clamp, hook, grapple," Danish klamme "a clamp, cramp," Old English clamm "fetter;" see clam (n.).
clamp (v.): "to fasten with a clamp," 1670s, from clamp (n.). Related: Clamped; clamping.
clamp-down (n.): also clampdown, 1940 in the figurative sense, from verbal phrase clamp down "use pressure to keep down" (1924). The verbal phrase in the figurative sense is recorded from 1941. See clamp (v.) + down (adv.).
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. As a noun form, there is awkward clandestinity (clandestineness apparently being a dictionary word).
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.
clang (n.): 1590s, from clang (v.).
clangor (n.): 1590s, from Latin clangor "sound of trumpets (Virgil), birds (Ovid), etc.," from clangere "to clang," echoic (cf. clang).
clangorous (adj.): 1712, from Medieval Latin clangorosus, from Latin clangor, or else from clangor + -ous. Related: Clangorously; clangorousness.
clank (v.): 1610s, perhaps echoic, perhaps a blend of clang (v.) and clink (v.), perhaps from a Low German source (cf. Middle Dutch clank, Dutch klank, Old High German klanc, Middle Low German klank, German Klang).
clank (n.): 1650s, from clank (v.). Reduplicated form clankety-clank attested from 1895.
clannish (adj.): "disposed to adhere closely to one another," 1769, from clan + -ish. Related: Clannishly; clannishness.
clansman (n.): 1810, "member of a clan," from genitive of clan + man (n.).
clap (v.): Old English clæppan "to throb, beat," common West Germanic, echoic (cf. Old Frisian klapa "to beat," Old Norse klappa, Old High German klaphon, German klappen, Old Saxon klapunga). Meaning "to strike or knock" is from c.1300. Meaning "to make a sharp noise" is late 14c. Of hands, to beat them together to get attention or express joy, from late 14c. To clap (someone) on the back is from 1520s. Related: Clapped; clapping.
clap (n.1): "loud noise," c.1200, from clap (v.). Of thunder, late 14c. Meaning "sudden blow" is from c.1400; meaning "noise made by slapping the palms of the hands together" is from 1590s.
clap (n.2): "gonorrhea," 1580s, of unknown origin, perhaps from Middle English clapper "rabbit-hole," from Old French clapoire (Modern French clapier), originally "rabbit burrow" (of uncertain origin), 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." Related: Clap-doctor.
clapboard (n.): 1520s, partial translation of Middle Dutch klapholt (borrowed into English late 14c. as clapholt), from klappen "to fit" + Low German holt "wood, board" (see holt). Cf. German Klappholz. Originally small boards of split oak, imported from northern Germany and cut by coopers to make barrel staves; the meaning "long, thin board used for roofing or to cover the exterior of wooden buildings" is from 1640s, American English.
clapper (n.): late 13c., agent noun from clap (v.). Meaning "tongue of a bell" is from late 14c. Old English had clipur. Meaning "hinged board snapped in front of a camera at the start of filming to synchronize picture and sound" is from 1940.
clapperclaw (v.): "to fight at arm's length with the hands and nails," 1590s, from clap (v.) + claw (v.). Related: Clapperclawed; clapperclawing.
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 claque "band of claqueurs," agent noun from claquer "to clap" (16c.), echoic (cf. clap (v.)). Modern sense of "band of political followers" is transferred from that of "organized applause at theater." Claqueur "audience memeber who gives pre-arranged responses in a theater performance" is in English from 1837.
This method of aiding the success of public performances is very ancient; but it first became a permanent system, openly organized and controlled by the claquers themselves, in Paris at the beginning of the nineteenth century. [Century Dictionary]
Clara: fem. personal name, from Latin Clara, from fem. of clarus "bright, shining, clear" (see clear (adj.) and cf. Claire). Derivatives include Clarisse, Clarice, Clarabel, Claribel. 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.): a thickened Roman 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 English meaning "red wine of Bordeaux" (excluding burgundy) first attested 1700. Used in pugilistic slang for "blood" from c.1600.
clarification (n.): 1610s, "act of clearing or refining" (especially of liquid substances), from French clarification, from Late Latin clarificationem (nominative clarificatio), noun of action from past participle stem of clarificare (see clarify). The meaning "statement revising or expanding an earlier statement but stopping short of a correction" is attested by 1969, originally in newspapers.
clarify (v.): early 14c., "make illustrious, make known," from Old French clarifiier "clarify, make clear, explain" (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).

Meaning "make clear, purify" is from early 15c. in English; intransitive sense of "grow or become clear" is from 1590s. Figurative sense of "to free from obscurity" is from 1823. Related: Clarified; clarifying.
clarinet (n.): 1768, from French clarinette (18c.), diminutive of clarine "little bell" (16c.), noun use of fem. of adjective clarin (which also was used as a noun, "trumpet, clarion"), from clair, cler (see clear (adj.)). Alternative form clarionet is attested from 1784.

The instrument, a modification of the medieval shawm, said to have been invented c.1700 by J.C. Denner of Nuremberg, Germany. A recognized orchestral instrument from c.1775. Ease of playing increased greatly with a design improvement from 1843 based on Boehm's flute.
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]
German Clarinet, Swedish klarinett, Italian clarinetto, etc. all are from French. Related: Clarinettist.
clarion (n.): "small, high-pitched type of trumpet," early 14c., from Old French clarion "(high-pitched) trumpet, bugle" and directly from Medieval Latin clarionem (nominative clario) "a trumpet," from Latin clarus "clear" (see clear (adj.)). Clarion call is attested from 1838.
Clarisse: fem. proper name, often a diminutive of Clara and its relatives. Also, "a nun of the order of St. Clare" (1790s); the Franciscan order also known as the Poor Clares (c.1600).
clarity (n.): c.1300, clarte "brightness," from Old French clarté "clarity, brightness," from Latin claritas "brightness, splendor," also, of sounds, "clearness;" figuratively "celebrity, renown, fame," from clarare "make clear," from clarus "clear" (see clear (adj.)). Modern form is early 15c., perhaps a reborrowing from Latin. Meaning "clearness" is from 1610s.
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 (v.): c.1500, "to make a loud, sharp sound," of imitative origin, or a blend of clap and crash. Cf. Dutch kletsen "splash, clash," German klatschen, Danish klaske "clash, knock about." Figurative sense, in reference to non-physical strife or battle, is first attested 1620s. Of things, "to come into collision," from 1650s; of colors, "to go badly together," first recorded 1894. Related: Clashed; clashing.
clash (n.): 1510s, "sharp, loud noise of collision," from clash (v.). Especially of the noise of conflicting metal weapons. Meaning "hostile encounter" is from 1640s; meaning "conflict of opinions, etc." is from 1781.
clasp (n.): c.1300, claspe, "metal catch or hook used to hold things together," perhaps a metathesis of clapse, and thus from or related to Old English clyppan "clasp" (see clip (v.2)).
clasp (v.): late 14c., from clasp (n.). Related: Clasped; clasping.
clasp-hook (n.): 1867, from clasp (n.) + hook (n.).
class (n.): c.1600, "group of students," from French classe (14c.), from Latin classis "a class, a 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" (a sense attested in English from 1650s), and thus akin to calare "to call (to arms)," from PIE root *kele- (2) "to shout" (see claim (v.)). In early use in English also in Latin form classis.

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. Meaning "high quality" is from 1847. Class-consciousness (1903) is from German klassenbewusst.