Online Etymology Dictionary

clit (n.): by 1969, slang shortening of clitoris.
clitellum (n.): "raised band around an earthworm," 1816, Modern Latin, from Latin clitellae "a pack-saddle," diminutive of *clitra "litter," from PIE *klei-tro-, from root *klei- (see lean (v.)). Related: Clitellar.
clitoral (adj.): 1887, from stem of clitoris + -al (1). Related: Clitorally. Alternative form clitorial is attested from 1879.
clitoridectomy (n.): 1866, from Greek klitorid- (see clitoris) + -ectomy. Originally in reference to a proposed cure for hysteria.
clitoris (n.): "erectile organ of female mammals," 1610s, coined in Modern Latin from Late Greek kleitoris, a diminutive, but the exact sense intended by the coiners is uncertain. Perhaps from Greek kleiein "to sheathe," also "to shut," in reference to its being covered by the labia minora. The related Greek noun form kleis has a second meaning of "a key, a latch or hook (to close a door);" see close (v.), and compare slot (n.2).

Alternatively, perhaps related to Greek kleitys, a variant of klitys "side of a hill," itself related to klinein "to slope," from the same root as climax (see lean (v.)), and with a sense of "little hill." Some ancient medical sources give a supposed Greek verb kleitoriazein "to touch or titillate lasciviously, to tickle" (compare German slang der Kitzler "clitoris," literally "the tickler"), but the verb is likely from the anatomy in this case.

The anatomist Mateo Renaldo Colombo (1516-1559), professor at Padua, claimed to have discovered it ("De re anatomica," 1559, p. 243). He called it amor Veneris, vel dulcedo "the love or sweetness of Venus." It had been known earlier to women.
cloaca (n.): 1650s, euphemism for "underground sewer," from Latin cloaca "public sewer, drain," from cluere "to cleanse," from PIE root *kleue- "to wash, clean" (source also of Greek klyzein "to dash over, wash off, rinse out," klysma "liquid used in a washing;" Lithuanian šluoju "to sweep;" Old English hlutor, Gothic hlutrs, Old High German hlutar, German lauter "pure, clear"). Use in biology, in reference to eliminatory systems of lower animals, is from 1834. Related: Cloacal (1650s); cloacinal (1857).
cloak (n.): late 13c., "long, loose outer garment," from Old North French cloque (Old French cloche, cloke) "travelling cloak," from Medieval Latin clocca "travelers' cape," literally "a bell," so called from the garment's bell-like shape (the word is thus a doublet of clock (n.1)). An article of everyday wear in England through 16c., somewhat revived 19c. as a fashion garment. Cloak-and-dagger (adj.) attested from 1848, said to be ultimately translating French de cape et d'épée, suggestive of stealthy violence and intrigue.
Other "cloak and dagger pieces," as Bouterwek tells us the Spaniards call their intriguing comedies, might be tried advantageously in the night, .... ["Levana; or the Doctrine of Education," English translation, London, 1848]
cloak (v.): c. 1500, from cloak (n.). Figuratively from 1540s. Related: Cloaked; cloaking.
cloak-room (n.): also cloakroom, 1852, from cloak (n.) + room (n.).
clobber (v.): 1941, British air force slang, probably related to bombing; possibly echoic. Related: Clobbered; clobbering. In late 19c. British slang the word principally had to do with clothing, as in clobber (n.) "clothes," (v.) "to dress smartly;" clobber up "to patch old clothes for reuse."
cloche (n.): type of bell jar, 1882, from French cloche "bell, bell glass" (12c.), from Late Latin clocca "bell" (see clock (n.1)). As a type of women's hat, recorded from 1907, so called from its shape.
clock (n.1): late 14c., clokke, originally "clock with bells," probably from Middle Dutch clocke (Dutch klok) "a clock," from Old North French cloque (Old French cloke, Modern French cloche "a bell"), from Medieval Latin (7c.) clocca "bell," which is probably from Celtic (compare Old Irish clocc, Welsh cloch, Manx clagg "a bell") and spread by Irish missionaries (unless the Celtic words are from Latin); ultimately of imitative origin.

Replaced Old English dægmæl, from dæg "day" + mæl "measure, mark" (see meal (n.1)). The Latin word was horologium; the Greeks used a water-clock (klepsydra, literally "water thief"). Image of put (or set) the clock back "return to an earlier state or system" is from 1862. Round-the-clock (adj.) is from 1943, originally in reference to air raids. To have a face that would stop a clock "be very ugly" is from 1886. (Variations from c. 1890 include break a mirror, kill chickens.)
I remember I remember
That boarding house forlorn,
The little window where the smell
Of hash came in the morn.
I mind the broken looking-glass,
The mattress like a rock,
The servant-girl from County Clare,
Whose face would stop a clock.

[... etc.; "The Insurance Journal," Jan. 1886]
clock (v.): "to time by the clock," 1883, from clock (n.1). The slang sense of "hit, sock" is 1941, originally Australian, probably from earlier slang clock (n.) "face" (1923). Related: Clocked; clocking.
clock (n.2): "ornament pattern on a stocking," 1520s, probably identical with clock (n.1) in its older sense and meaning "bell-shaped ornament."
clock-radio (n.): 1946, from clock (n.1) + radio (n.).
clock-watcher (n.): "employee habitually prompt in leaving," 1887, from clock (n.1) + agent noun from watch (v.). Related: Clock-watching. Compare earlier tell-clock "idler" (c. 1600).
clockwise (adv.): also clock-wise, "in the direction of the hands of a clock," 1879, from clock (n.1) + wise (n.).
clockwork (n.): also clock-work, 1660s, "mechanism of a clock," from clock (n.1) + work (n.). Figurative sense of "anything of unvarying regularity" is recorded earlier (1620s).
clod (n.): "lump of earth or clay," Old English clod- (in clodhamer "the fieldfare," a kind of thrush, literally "field-goer"), from Proto-Germanic *kludda-, from PIE *gleu- (see clay).

Synonymous with collateral clot until meaning differentiated 18c. Meaning "person" ("mere lump of earth") is from 1590s; that of "blockhead" is from c. 1600 (compare clodpate, clodpoll, etc.). It also was a verb in Middle English, meaning both "to coagulate, form into clods" and "to break up clods after plowing."
cloddish (adj.): 1844, from clod (n.) + -ish. Related: Clodishly; clodishness.
clodhopper (n.): 1690s, slang, "one who works on plowed land, a rustic," from clod (n.) + agent noun from hop (v.). Compare in a similar sense clod-breaker, clod-crusher; in this word perhaps a play on grasshopper. Sense extended by 1836 to the shoes worn by such workers.
clog (n.): early 14c., clogge "a lump of wood," origin unknown. Also used in Middle English of large pieces of jewelry and large testicles. Compare Norwegian klugu "knotty log of wood." Meaning "anything that impedes action" is from 1520s. The sense of "wooden-soled shoe" is first recorded late 14c.; they were used as overshoes until the introduction of rubbers c. 1840. Originally all wood (hence the name), later wooden soles with leather uppers for the front of the foot only. Later revived in fashion (c. 1970), primarily for women. Clog-dancing is attested from 1863.
clog (v.): late 14c., "hinder," originally by fastening a block of wood to something, from clog (n.). Meaning "choke up with extraneous matter" is 17c. Related: Clogged; clogging.
cloison (n.): "a partition, a dividing band," 1690s, from French cloison, from Vulgar Latin *clausionem (nominative *clausio), noun of action from past participle stem of Latin claudere "to close, shut" (see clause). Related: Cloisonnage.
cloisonne (adj.): "divided into compartments," 1863, from French cloisonné, from cloison "a partition" (12c., in Old French, "enclosure"), from Provençal clausio, from Vulgar Latin *clausio, noun of action from past participle stem of claudere "to close, shut" (see clause).
cloister (n.): early 13c., from Old French cloistre "monastery, convent; enclosure" (12c., Modern French cloître), from Medieval Latin claustrum "portion of monastery closed off to laity," from Latin claustrum (usually in plural, claustra) "place shut in, enclosure; bar, bolt, means of shutting in," from past participle stem of claudere (see close (v.)).

"The original purpose of cloisters was to afford a place in which the monks could take exercise and recreation" [Century Dictionary]. Spelling in French influenced by cloison "partition." Old English had clustor, clauster in the sense "prison, lock, barrier," directly from Latin, and compare, from the same source, Dutch klooster, German Kloster, Polish klasztor.
cloister (v.): c. 1400 (implied in cloistered), from cloister (n.). Figurative use from c. 1600. Related: Cloistered; cloistering.
cloistral (adj.): c. 1600, from cloister + -al (1).
clomp (v.): "to walk as with clogs," 1829, probably a variant of clump (v.). Related: Clomped; clomping.
clone (n.): 1903, in botany, from Greek klon "a twig, spray," related to klados "sprout, young branch, offshoot of a plant," possibly from PIE root *kel- (1) "to strike, cut" (see holt). Figurative use by 1978.
clone (v.): 1959, from clone (n.). Related: Cloned; cloning. Extension to genetic duplication of animals and human beings is from 1970.
clonk (v.): 1930, imitative. Related: Clonked; clonking.
clonus (n.): "violent muscular spasms," 1817, from Modern Latin, from Greek klonos "turmoil, any violent motion; confusion, tumult, press of battle," from PIE *kel- "to drive, set in motion." Related: Clonic; clonicity.
Clootie (n.): also Clutie, "the devil," 1785, Scottish, literally "hoofed," from cloot "hoof, division of a hoof," from Old Norse klo "claw" (see claw (n.)).
clop (v.): 1897, echoic of the sound of boots or hoofs on the ground. Related: Clopped; clopping.
Cloris: fem. proper name, from Chloris, Latin form of Greek Khloris, goddess of flowers (later identified with Roman Flora), literally "greenness, freshness," poetic fem. of khloros "greenish-yellow, fresh," related to khloe "young green shoot" (see Chloe).
close (v.): c. 1200, "to shut, cover in," from Old French clos- (past participle stem of clore "to shut, to cut off from"), 12c., from Latin clausus, past participle of claudere "to shut, close; to block up, make inaccessible; put an end to; shut in, enclose, confine" (always -clusus, -cludere in compounds).

The Latin word might be from the possible PIE root *klau- "hook, peg, crooked or forked branch" (used as a bar or bolt in primitive structures); source also of Latin clavis "key," clavus "nail," claustrum "bar, bolt, barrier," claustra "dam, wall, barricade, stronghold;" Greek kleidos (genitive) "bar, bolt, key," klobos "cage;" Old Irish clo "nail," Middle Irish clithar "hedge, fence;" Old Church Slavonic ključi "hook, key," ključiti "shut;" Lithuanian kliuti "to catch, be caught on," kliaudziu "check, hinder," kliuvu "clasp, hang;" Old High German sliozan "shut," German schließen "to shut," Schlüssel "key."

Also partly from Old English beclysan "close in, shut up." Intransitive sense "become shut" is from late 14c. Meaning "draw near to" is from 1520s. Intransitive meaning "draw together, come together" is from 1550s, hence the idea in military verbal phrase close ranks (mid-17c.), later with figurative extensions. Meaning "bring to an end, finish" is from c. 1400; intransitive sense "come to an end" is from 1826. Of stock prices, from 1860. Meaning "bring together the parts of" (a book, etc.) is from 1560s. Related: Closed; closing.
close (adj.): late 14c., "strictly confined," also "secret," from Old French clos "confined; concealed, secret; taciturn" (12c.), from Latin clausus "close, reserved," past participle adjective from claudere "stop up, fasten, shut" (see close (v.)); main sense shifting to "near" (late 15c.) by way of "closing the gap between two things." Related: Closely.

Meaning "narrowly confined, pent up" is late 14c. Meaning "near" in a figurative sense, of persons, from 1560s. Meaning "full of attention to detail" is from 1660s. Of contests, from 1855. Close call is from 1866, in a quotation in an anecdote from 1863, possibly a term from the American Civil War; close shave in the figurative sense is 1820, American English. Close range is from 1814. Close-minded is attested from 1818. Close-fisted "penurious, miserly" is from c. 1600.
close (n.): late 14c., "act of closing, conclusion, termination," from close (v.). Also in early use "enclosure, enclosed space" (late 13c.), from Old French clos, noun use of past participle.
close (adv.): "tightly, with no opening or space between," from close (adj.).
close quarters: 1753, originally nautical, also close-fights, "bulkheads fore and aft for men to stand behind in close engagements to fire on the enemy," it reflects the confusion of close (v.) and close (adj.); "now understood of proximity, but orig. 'closed' space on ship-board where last stand could be made against boarders" [Weekley]. Compare also closed-minded, a variant of close-minded attested from 1880s, with a sense of "shut" rather than "tight."
close-up (n.): 1913, in photography, etc.; see close (adv.) + up (adv.).
closed (adj.): c. 1200, past participle adjective from close (v.). Closed circuit is attested from 1827; closed shop in union sense from 1904; closed system first recorded 1896 in William James.
closely (adv.): 1550s, "secretly," from close (adj.) + -ly (2). From 1560s as "compactly," 1590s as "so as to enclose;" 1630s as "nearly."
closeness (n.): mid-15c., "confined condition," from close (adj.) + -ness. Meaning "stuffiness" (of air) is from 1590s; meaning "nearness" is from 1716.
closer (n.): "one who closes" anything, 1610s, agent noun from close (v.).
closet (n.): late 14c., from Old French closet "small enclosure, private room," diminutive of clos "enclosure," from Latin clausum "closed space, enclosure, confinement," from neuter past participle of claudere "to shut" (see close (v.)). In Matt. vi:6 it renders Latin cubiculum "bedchamber, bedroom," Greek tamieion "chamber, inner chamber, secret room;" thus originally in English "a private room for study or prayer." Modern sense of "small side-room for storage" is first recorded 1610s.

The adjective is from 1680s, "private, secluded;" meaning "secret, unknown" recorded from 1952, first of alcoholism, but by 1970s used principally of homosexuality; the phrase come out of the closet "admit something openly" first recorded 1963, and lent new meanings to the word out.
closet (v.): "shut up as in a closet" (originally usually for purposes of concealment or private consultation), 1680s, from closet (v.). Related: Closeted; closeting.
closing (n.): late 14c., "act of closing; that which closes," verbal noun from close (v.).
closure (n.): late 14c., "a barrier, a fence," from Old French closure "enclosure; that which encloses, fastening, hedge, wall, fence," also closture "barrier, division; enclosure, hedge, fence, wall" (12c., Modern French clôture), from Late Latin clausura "lock, fortress, a closing" (source of Italian chiusura), from past participle stem of Latin claudere "to close" (see close (v.)). Sense of "act of closing, bringing to a close" is from early 15c. In legislation, especially "closing or stopping of debate." Sense of "tendency to create ordered and satisfying wholes" is 1924, from Gestalt psychology.