Online Etymology Dictionary

claudication (n.): 1550s, from Latin claudicationem, noun of action from past participle stem of claudicare, from claudus "limping, halting, lame." Related: Claudicant; claudicare.
Claudius: name of two Roman gentes, perhaps related to claudus "lame," which is of unknown origin. The masc. proper name Claude is a derivative.
clause (n.): early 13c., from Old French clause, from Medieval Latin clausa "conclusion," from Latin clausula "the end, a closing, termination," also "end of a sentence or a legal argument," from clausus, fem. past participle of claudere "to close, to shut, to conclude" (see close (v.)). Meaning "article or section of text" is recorded from c.1300 as the sense of "ending" gradually faded from the word. Related: Clausal.
claustrophobia (n.): coined 1879 (first in article by B. Ball in "British Medical Journal") in Modern Latin, from Latin claustrum "a bolt, a means of closing, a place shut in" (in Medieval Latin "cloister," hence claustral), past participle of claudere "to close" (see close (v.)) + -phobia "fear."
claustrophobic (adj.): 1889, from claustrophobia + -ic. As a noun, "person who has claustrophobia," it is recorded from 1953.
clavichord (n.): mid-15c., from Medieval Latin clavicordium, from Latin clavis "a key" (see slot (n.2)) + chorda "a string" (see cord).
clavicle (n.): 1610s, from Middle French clavicule "collarbone," also "small key," from Medieval Latin clavicula (used c.980 in a translation of Avicenna), from Latin clavicula, literally "small key, bolt," diminutive of clavis "key" (see slot (n.2)); a loan-translation of Greek kleis "key, collarbone." So called supposedly from its function as the "fastener" of the shoulder. Related: Clavicular.
clavier (n.): 1708, "keyboard," from French clavier, originally "a key-bearer," from Latin clavis "key" (see slot (n.2)). The French word also is the source of German Klavier, Dutch klavier, etc. The German word is the direct source of the name of the musical instrument, a sense attested from 1845 in English.
claw (n.): Old English clawu, earlier clea, "claw, talon," from Proto-Germanic *klawo (cf. Old Frisian klawe "claw, hoe," Middle Dutch klouwe, Old High German klawa, German Klaue), from PIE *g(e)l-eu- from root *gel- "to make round, clench."
claw (v.): Old English clawian, from the same root as claw (n.). Related: Clawed; clawing.
clay (n.): Old English clæg "stiff, sticky earth; clay," from West Germanic *klaijaz (cf. Old High German kliwa "bran," German Kleie, Old Frisian klai "clay," Old Saxon klei, Middle Dutch clei, Danish klæg; Old English clæman, Old Norse kleima, Old High German kleiman "to cover with clay"), from PIE root *glei- "to stick together" (cf. Greek gloios "sticky matter;" Latin gluten "glue;" Old Church Slavonic glina "clay," glenu "slime, mucus;" Old Irish glenim "I cleave, adhere"). Clay pigeon is from 1888.
claymore (n.): 1772, "two-edged broadsword of ancient Scottish Highlanders," from Gaelic claidheamh mor "great sword," from claidheb "sword" (from PIE root *kel- "to strike;" see holt) + mor "great" (see more). An antiquarian word made familiar again by Scott's novels; modern military application to pellet-scattering anti-personnel mine is first attested 1962.
clean (adj.): Old English clæne "clean, pure," from West Germanic *klainoz "clear, pure" (cf. Old Saxon kleni "dainty, delicate," Old Frisian klene "small," Old High German kleini "delicate, fine, small," German klein "small;" English preserves the original Germanic sense), from PIE root *gel- "to gleam" (cf. Greek glene "eyeball," Old Irish gel "bright").

Old English also had an adverbial form, "clearly, fully, entirely." As an adjective, replaced in higher senses by clear, pure, but as a verb (mid-15c.) it has largely usurped what once belonged to cleanse. The adjective clean in the sense of "innocent" is from c.1300; that of "not lewd" is from 1867; that of "free of drug addiction" is 1950s.
clean (v.): mid-15c., from clean (adj.). Related: Cleaned; cleaning.
cleaner (n.): mid-15c., agent noun from clean (v.). To take (someone) to the cleaners "get all of (someone's) money" is from 1921.
cleanliness (n.): early 15c., from cleanly + -ness.
Cleanliness is indeed next to godliness. [John Wesley, Sermon "On Dress," c.1791]
cleanly (adj.): Old English clænlic "pure, excellent," from clæne (see clean (adj.)).
cleanse (v.): Old English clænsian "to cleanse, purge, purify, chasten, justify," from West Germanic *klainson, from *klainoz (see clean). Despite its modern spelling (16c.), it retains its Middle English pronunciation. Related: Cleansed; cleansing.
cleanser (n.): Old Engilsh clænsere "priest," agent noun from clænsian (see cleanse). Meaning "thing that cleanses" is from late 14c.
cleansing (n.): Old English clænsunge "cleansing, purifying, castigation; chastity, purity," verbal noun from the root of cleanse. As a present participle adjective, attested from c.1300.
cleanup (n.): also clean-up, 1856, "act of cleaning up," from clean + up. Meaning "a profit" is recorded from 1878. Verbal phrase clean up "make a large profit" is from 1929. The adjective, in the baseball sense, is recorded by 1910.
clear (adj.): late 13c., "bright," from Old French cler "clear" (of sight and hearing), "light, bright, shining; sparse" (12c., Modern French clair), from Latin clarus "clear, loud," of sounds; in transferred use, of sights, "bright, distinct;" also "illustrious, famous, glorious" (source of Italian chiaro, Spanish claro), from PIE *kle-ro-, from root *kele- (2) "to shout" (see claim (v.)).

The sense evolution involves an identification of the spreading of sound and the spreading of light (cf. English loud, used of colors; German hell "clear, bright, shining," of pitch, "distinct, ringing, high"). Of the weather, from late 14c.; of meanings or explanations, c.1300. (An Old English word for this was sweotol "distinct, clear, evident".) Sense of "free from encumbrance," apparently nautical, developed c.1500. Phrase in the clear attested from 1715.
clear (v.): late 14c., "to enlighten," from clear (adj.). Meaning "to leap clear over" is first attested 1791. Related: Cleared; clearing.
clearance (n.): 1560s, "action of clearing," from clear (v.) + -ance. Meaning "a clear space" is from 1788. Meaning "approval, permission" (especially to land or take off an aircraft) is from 1944, American English; national security sense recorded from 1948. Clearance sale attested by 1843.
clearing (n.): "land cleared of wood," 1670s, from noun use of present participle of clear (v.).
clearinghouse (n.): 1832, from clearing + house (n.). The original was established 1775 in London by the bankers for the adjustment of their mutual claims for checks and bills; later the word was extended to similar institutions.
clearly (adv.): c.1300, of vision and speech, from clear (adj.) + -ly (2). Meaning "evidently" is from 1560s; as a parenthetical expression in argument, "it is clear," recorded from 1867.
cleat (n.): c.1300, clete "wedge," from Old English *cleat "a lump," from West Germanic *klaut "firm lump" (cf. Middle Low German klot, klute, Middle Dutch cloot, Dutch kloot, Old High German kloz, German kloß "clod, dumpling"), from PIE root *gleu- "to form into a ball" (see clay). In Middle English, a wedge of wood bolted to a spar, etc., to keep it from slipping. Meaning "thin metal plate for shoes, etc." is c.1825.
cleavage (n.): 1816, in geology, "action of splitting (rocks or gems) along natural fissures," from cleave (v.1) + -age. General meaning "action or state of cleaving or being cleft" is from 1867. The sense of "cleft between a woman's breasts in low-cut clothing" is first recorded 1946, when it was defined in a "Time" magazine article [Aug. 5] as the "Johnston Office trade term for the shadowed depression dividing an actress' bosom into two distinct sections."
cleave (v.1): "to split," Old English cleofan "to split, separate" (class II strong verb, past tense cleaf, past participle clofen), from Proto-Germanic *kleubanan (cf. Old Saxon klioban, Old Norse kljufa, Danish klöve, Dutch kloven, Old High German klioban, German klieben "to cleave, split"), from PIE root *gleubh- "to cut, slice" (see glyph).

Past tense form clave is recorded in Northern writers from 14c. and was used with both verbs (see cleave (v.2)), apparently by analogy with other Middle English strong verbs. Clave was common to c.1600 and still alive at the time of the KJV; weak past tense cleaved for this verb also emerged in 14c.; cleft is still later. The past participle cloven survives, though mostly in compounds.
cleave (v.2): "to adhere," Old English clifian, from West Germanic *klibajanan (cf. Old Saxon klibon, Old High German kliban, Dutch kleven, Old High German kleben, German kleben "to stick, cling"), from PIE *gloi- "to stick" (see clay). The confusion was less in Old English when cleave (v.1) was a class 2 strong verb; but it has grown since cleave (v.1) weakened, which may be why both are largely superseded by stick (v.) and split (v.).
cleaver (n.): late 15c., "one who splits," agent noun from cleave (v.1). Originally "one who splits boards with a wedge instead of sawing;" attested as a surname from early 14c. Meaning "butcher's chopper" is from 1570s.
This last ["Marrowbones and Cleaver"] is a sign in Fetter Lane, originating from a custom, now rapidly dying away, of the butcher boys serenading newly married couples with these professional instruments. Formerly, the band would consist of four cleavers, each of a different tone, or, if complete, of eight, and by beating their marrowbones skilfully against these, they obtained a sort of music somewhat after the fashion of indifferent bell-ringing. When well performed, however, and heard from a proper distance, it was not altogether unpleasant. ... The butchers of Clare market had the reputation of being the best performers. ... This music was once so common that Tom Killigrew called it the national instrument of England. [Larwood & Hotten, "The History of Signboards from the Earliest Times to the Present Day," London, 1867]
clechy (n.): also cleche, from French cléché (17c.), from Latin *clavicatus "key-holed," or clavicella "little key," from clavis "key" (see slot (n.2)).
cledonism (n.): "avoidance of words deemed unlucky," from Greek kledon "omen," also "report, fame" (see claim (v.)) + -ism.
clef (n.): 1570s in a musical sense, from Middle French clef (12c.) "key, musical clef, trigger," from a figurative or transferred use of classical Latin clavis, which had only the literally sense "key" (see slot (n.2)). In the Middle Ages, the Latin word was used in the Guidonian system for "the lowest note of a scale," which is its basis (see keynote). The most common is the treble or G-clef, denoting the G above middle C on the piano.
cleft (n.): 1570s, altered (by influence of cleft, new weak past participle of cleave (v.1)), from Middle English clift (early 14c.), from Old English geclyft (adj.) "split, cloven," from Proto-Germanic *kluftis (cf. Old High German and German kluft, Danish kløft "cleft"), from PIE *gleubh- (see glyph).
clematis (n.): plant genus, 1550s, from Latin Clematis, from Greek klematis, name of some kind of climbing or trailing plant (probably the periwinkle), diminutive of klema "vine-branch, shoot or twig broken off" (for grafting), from klan "to break" (see clastic).
clemency (n.): 1550s, "mildness or gentleness shown in exercise of authority," from Latin clementia "calmness, gentleness," from clemens "calm, mild," related to -clinare "to lean" (see lean (v.)) + participial suffix -menos (also in alumnus).

For sense evolution, cf. inclined in secondary meaning "disposed favorably." Meaning "mildness of weather or climate" is 1660s (a sense also in Latin); clement (adj.) is older in both senses, late 15c. and 1620s respectively, but now is used only in negation and only of the weather.
clement (adj.): mid-15c., "mild," of persons (attested from early 13c. as a surname), from Old French clement, from Latin clementem (nominative clemens) "mild, placid, gentle" (see clemency). Of weather, 1620s. Taken as a name by several early popes and popular in England as a masculine given name from mid-12c., also in fem. form Clemence.
clementine (n.): "cross between tangerine and sour orange," 1926, from French clémentine (1902). Originally an accidental hybrid said to have been discovered by (and named for) Father Clement Rodier in the garden of his orphanage in Misserghin, near Oran, Algeria. Introduced into U.S. and grown at Citrus Research Center in Riverside, Calif., as early as 1909. Earlier the word was used as an adjective in reference to other Clements, especially of the edition of the Vulgate issued due to Pope Clement V (1309-14).
clench (v.): Old English beclencan "to hold fast, make cling," causative of clingan (see cling); cf. stink/stench. Related: Clenched; clenching. The noun is attested from 1799.
Cleopatra: common name of sister-queens in Egypt under the Ptolemaic Dynasty. The name is Greek, probably meaning "key to the fatherland," from kleis "key" + patris. The famous queen was the seventh of that name.
cleptomaniac (n.): variant of kleptomaniac (see kleptomania).
clerestory (n.): early 15c., probably from clere "clear," in a sense "light, lighted" (see clear), and story, though this sense of that word is not otherwise found so early. Originally the upper part of the nave, transepts, and choir of a large church.
clergy (n.): c.1200, clergie "office or dignity of a clergyman," from two Old French words: 1. clergié "clerics, learned men," from Medieval Latin clericatus, from Late Latin clericus (see clerk); 2. clergie "learning, knowledge, erudition," from clerc, also from Late Latin clericus. Meaning "persons ordained for religious work" is from c.1300.
clergyman (n.): 1570s, from clergy + man (n.). Clergywoman is 1670s.
cleric (n.): 1620s, from Late Latin clericus "clergyman, priest" (4c.), literally "belonging to the clerus;" from Ecclesiastical Greek klerikos, "belonging to the clergy," originally "pertaining to an inheritance," but in Christian use "pertaining to the ecclesiastical order," from kleros "lot, allotment, piece of land, heritage," which was in Church use from 2c. (see clerk). A word taken up in English after clerk took its modern meaning.
clerical (adj.): 1590s, "pertaining to the clergy," from cleric + -al (1). Meaning "pertaining to clerks" is from 1798.
clerihew (n.): humorous verse form, 1928, from English humorist Edmund Clerihew Bentley (1875-1956), who described it in a book published 1906 under the name E. Clerihew.
clerk (n.): Old English clerc, from Late Latin clericus "a priest," from Greek klerikos (adj.) in church jargon "of the clergy," derived from kleros "lot, inheritance" (originally "a shard or wood chip used in casting lots," related to klan "to break;" see holt), used by early Greek Christians for matters relating to ministry based on Deut. xviii:2 reference to Levites as temple assistants: "Therefore shall they have no inheritance among their brethren: the Lord is their inheritance," kleros being used as a translation of Hebrew nahalah "inheritance, lot."

If the word choice was meant to remind clerics of anything, however, the reminder was lost with the knowledge of ancient Greek. Or else it is from the use of the word in Acts i:17. Modern bureaucratic usage is from c.1500, a reminder of the dark ages when clergy alone could read and write. Related: Clerkship.