Online Etymology Dictionary

clout (n.): Old English clut "lump of something," also "patch of cloth put over a hole to mend it," from Proto-Germanic *klutaz (cf. Old Norse klute "kerchief," Danish klud "rag, tatter," Frisian klut "lump," Dutch kluit "clod, lump"); perhaps related to clot. In later use "a handkerchief," also "a woman's sanitary napkin." Sense of "a blow" is from early 14c., first in a verbal sense, "to beat, strike;" perhaps somehow from the "patch" sense. Sense of "personal influence" is 1958, on the notion of "punch, force."
clove (n.1): dried flowerbud of a tropical tree, used as a spice, late 15c., earlier clowes (14c.), from Anglo-French clowes de gilofre (c.1200), Old French clou de girofle "nail of gillyflower," so called from its shape, from Latin clavus "a nail" (see slot (n.2)). For second element, see gillyflower. The two cloves were much confused in Middle English.
clove (n.2): "slice of garlic," Old English clufu, from Proto-Germanic *klubo "cleft, thing cloven," from PIE *gleubh- "to tear apart, cleave" (see cleave (v.1)). Cognates mostly in compounds that translate as "clove-leek;" e.g. Old Saxon clufloc, Old High German chlobilouh. Dutch knoflook, German knoblauch are by dissimilation.
cloven (adj.): past participle adjective from cleave (v.1).
clover (n.): Old English clafre, from Proto-Germanic *klaibron (cf. Middle Low German klever, Middle Dutch claver, Dutch klaver, Old Saxon kle, Old High German kleo, German Klee "clover"), of uncertain origin.

Liberman suggests it is probably from West Germanic *klaiwaz- "sticky pap" (see clay), and adds, "The sticky juice of clover was the base of the most popular sort of honey." First reference in English to the suposed luck of a four-leaf clover is from c.1500. To be in clover "live luxuriously" is 1710, "clover being extremely delicious and fattening to cattle" [Johnson].
cloverleaf (n.): 1882, from clover + leaf (n.). Highway interchange sense attested by 1933.
Clovis: type of prehistoric stone spearpoints, 1943, from Clovis, New Mexico, U.S., near which place they were found. The town is said to have been named for the Frankish king Clovis (Latinized from Frankish Chlodovech, from Germanic masc. proper name *hluda-wigaz "famous in battle," cognate with Ludwig and Louis).
clowder: 1811, variant of clutter.
clown (n.): 1560s, also cloyne, "rustic, boor, peasant," origin uncertain. Perhaps from Scandinavian dialect (cf. Icelandic klunni "clumsy, boorish fellow;" Swedish kluns "a hard knob, a clumsy fellow"), or akin to North Frisian klönne "clumsy person," or, less likely, from Latin colonus "colonist, farmer." Meaning "fool, jester" is c.1600. "The pantomime clown represents a blend of the Shakes[pearean] rustic with one of the stock types of the It. comedy" [Weekley]. Meaning "contemptible person" is from 1920s.
clown (v.): c.1600, from clown (n.); colloquial sense of "to behave inappropriately" (e.g. clown around, 1932) attested by 1928, perhaps originally in music performance slang. Related: Clowned; clowning.
cloy (v.): "weary by too much, fill to loathing, surfeit," 1520s, from Middle English cloyen "hinder movement, encumber" (late 14c.), a shortening of accloyen (early 14c.), from Old French encloer "to fasten with a nail, grip, grasp," figuratively "to hinder, check, stop, curb," from Late Latin inclavare "drive a nail into a horse's foot when shoeing," from Latin clavus "a nail" (see slot (n.2)). The meaning "fill to a sateity, overfill" is attested for accloy from late 14c.
Accloye is a hurt that cometh of shooing, when a Smith driveth a nail in the quick, which make him to halt. [Edward Topsell, "The History of Four-footed Beasts," 1607]
Related: Cloyed; cloying.
cloze: 1953, in psychological writing, abstracted from closure.
club (n.): c.1200, "thick stick used as a weapon," from Old Norse klubba "cudgel," from Proto-Germanic *klumbon, related to clump (n.). Old English words for this were sagol, cycgel. Specific sense of "bat used in games" is from mid-15c. The social club (1660s) apparently evolved from this word 17c. from the verbal sense "gather in a club-like mass" (1620s), then "association of people" (1640s).
I got a good mind to join a club and beat you over the head with it. [Rufus T. Firefly]
Club sandwich first recorded 1903; club soda is 1877, originally a proprietary name. The club at cards (1560s) is the right name for the suit (Spanish basto, Italian bastone), but the pattern adopted on English cards is the French trefoil.
club (v.): "to hit with a club," 1590s, from club (v.). Meaning "gather in a club-like mass" is from 1620s. Related: Clubbed; clubbing.
CLUB, verb (military). -- In manoeuvring troops, so to blunder the word of command that the soldiers get into a position from which they cannot extricate themselves by ordinary tactics. [Farmer & Henley]
club-foot (n.): also clubfoot, 1530s, from club (n.) + foot (n.).
club-house (n.): also clubhouse, 1818, from club (n.) in the associative sense + house (n.). Clubhouse lawyer is baseball slang by 1940s.
cluck (v.): Old English cloccian originally echoic. Cf. Turkish culuk, one of the words for "turkey;" Greek klozan, Latin glocire, German glucken. Related: Clucked; clucking.
cluck (n.): 1703, from cluck (v.). Slang meaning "stupid person" (turkeys are famously foolish) is from 1927.
clue (n.): 1590s, phonetic variant of clew "a ball of thread or yarn," with reference to the one Theseus used as a guide out of the Labyrinth. The purely figurative sense of "that which points the way" is from 1620s.
clue (v.): "to inform someone of the important facts," usually with in, 1934, from clue (n.). Related: Clued; cluing. Earlier in 19c. sailing, "to man clue lines so as to force a yard down on the mast cap."
clueless (adj.): 1862, "trackless," from clue (n.) + -less. Meaning "ignorant, uninformed" is from 1943, said to be RAF slang from 1930s. Student slang use by 1985 is perhaps an independent extension along the same line. Related: Cluelessly; cluelessness.
clump (n.): 1580s, "lump, cluster of trees," from Middle English clompe "a lump" (c.1300), from Dutch klomp "lump, mass," or Middle Low German klumpe "clog, wooden shoe." Old English had clympre "lump, mass of metal."
clump (v.2): "walk heavily," 1660s, imitative. Related: Clumped; clumping.
clump (v.1): "to heap or gather in clumps" (transitive), 1824, from clump (n.). Related: Clumped; clumping. Intransitive sense "to form a clump or clumps" is recorded from 1896.
clumsy (adj.): 1590s, from Middle English clumsid "numb with cold" (14c.), past participle of clumsen "to benumb," from Old Norse klumsa, intensive of kluma "to make motionless." Related: Clumsily; clumsiness.
clung: Old English clungen, past tense and past participle of cling.
clunk (v.): 1796, "to make the sound of a cork being pulled from a bottle;" imitative. This was the main sense through most of 19c. Meaning "to hit, strike" is attested from 1940s. Related: Clunked; clunking.
clunker (n.): "anything inferior," 1940s, agent noun from clunk. Specific sense of "old car" was in use by 1951 (clunk in the sense "old worn-out machine" is from 1940s).
clunky (adj.): "blocky, ungraceful," by 1968 (when it was the name of a style of women's shoe), from clunk + -y (2). Related: Clunkily; clunkiness.
cluster (n.): Old English clyster "cluster," probably from the same root as clot (n.).
cluster (v.): late 14c., from cluster (n.). Related: Clustered; clustering.
clusterfuck (n.): "bungled or confused undertaking," 1969, U.S. military slang, from cluster + fuck, probably in the "bungle" sense. Earlier the compound meant "orgy" (1966).
clutch (v.): Old English clyccan "bring together, bend (the fingers), clench," from PIE *klukja- (cf. Swedish klyka "clamp, fork;" related to cling). Influenced in meaning by Middle English cloke "a claw." Related: Clutched; clutching.
clutch (n.3): "nest" in reference to chickens, eggs, 1721, from clekken "to hatch" (c.1400). Cf. batch/bake. Probably from a Scandinavian source (e.g. Old Norse klekja "to hatch"), perhaps of imitative origin (cf. cluck).
clutch (n.1): "claw, grip, grasp," c.1300, from cloche "claw," from cloke (c.1200), related to clucchen, clicchen (see clutch (v.)). Meaning "grasping hand" (1520s) led to that of "tight grasp" (1784). Related: Clutches.
clutch (n.2): mechanical part, 1814, from clutch (v.), with the "seizing" sense extended to "device for bringing working parts together." Originally of mill-works, first used of motor vehicles 1899. Meaning "moment when heroics are required" is attested from 1920s.
clutter (v.): 1550s, "to collect in heaps," variant of clotern "to form clots, to heap on" (c.1400); related to clot. Sense of "to litter" is first recorded 1660s. As a noun, from 1570s; the "litter" sense is from 1660s. Related: Cluttered; cluttering.
Clyde: masc. proper name, from the family name, from the region of the Clyde River in Scotland (see Clydesdale). Most popular in U.S. for boys c.1890-1910, falling off rapidly thereafter, hence probably its use in 1940s teenager slang for "a square, one not versed in popular music or culture."
Clydesdale: "breed of heavy draught horses," 1786, so called because they were bred in the valley of the Clyde in Scotland. The river name is perhaps literally "cleansing," from a Celtic root akin to Latin cloaca (see cloaca).
clyster (n.): from French clystère (13c.), from Latin, from Greek klyster, from klyzein "to wash out" (see cloaca).
Clytaemnestra: also Clytemnestra, wife and murderess of Agamemnon, from Greek Klytaimnestra, from klytos "celebrated, heard of" (see loud) + mnester "wooer, suitor," literally "willing to mind, mindful of," related to mnasthai "to remember," from PIE root *men- "to think" (see mind (n.)).
Cnidaria (n.): phylum of stinging invertebrates, from Modern Latin cnida, from Greek knide "nettle," from stem of knizein "to scratch scrape." Related: Cnidarian.
co-: in Latin, the form of com- in compounds with stems beginning in vowels and h- and gn- (see com-). Taken in English from 17c. as a living prefix meaning "together, mutually, in common," and used promiscuously with native words and Latin-derived words not beginning with vowels, sometimes even with words already having it (e.g. co-conspiritor).
co-ed (n.): also coed, 1886, American English, (first in Louisa Mae Alcott's "Jo's Boys"); short for "co-educational system;" 1889 as an adjective, short for coeducational; 1893 as a noun meaning "girl or woman student at a co-educational institution."
co-op (n.): 1861, abbreviation of cooperative. The hyphen is needed to avoid confusion with coop (n.).
co-opt (v.): 1650s, "to select (someone) for a group or club by a vote of members," from Latin cooptare "to elect, to choose as a colleague or member of one's tribe," from com- "together" (see com-) + optare "choose" (see option (n.)). For some reason this defied the usual pattern of Latin-to-English adaptation, which should have yielded *cooptate. Sense of "take over" is first recorded c.1953. Related: Co-opted; co-opting.
co-ordinate: see coordinate.
co-star: also costar, 1919 as a verb; 1926 as a noun, from co- + star (v.).
co.: abbreviation of company, attested by 1670s.
coach (n.): 1550s, "large kind of carriage," from Middle French coche (16c.), from German kotsche, from Hungarian kocsi (szekér) "(carriage) of Kocs," village where it was first made. In Hungary, the thing and the name for it date from 15c., and forms are found in most European languages (e.g. Spanish and Portuguese coche, Italian cocchino, Dutch koets). Applied to railway cars 1866, American English. Sense of "economy or tourist class" is from 1949. Meaning "instructor/trainer" is c.1830 Oxford University slang for a tutor who "carries" a student through an exam; athletic sense is 1861.