Online Etymology Dictionary

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.
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; figuratively "manifest, plain, evident," 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 (compare English loud, used of colors; German hell "clear, bright, shining," of pitch, "distinct, ringing, high"). Of complexion, from c. 1300; of the weather, from late 14c.; of meanings or explanations, "manifest to the mind, comprehensible," 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-sighted is from 1580s (clear-eyed is from 1529s); clear-headed is from 1709.
clear (v.): late 14c., "to fill with light," from clear (adj.). Of weather, from late 14c. Meaning "make clear in the mind" is mid-15c., as is sense of "to remove what clouds." Meaning "to prove innocent" is from late 15c. Meaning "get rid of" is from 1530s.

Meaning "to free from entanglement" is from 1590s; that of "pass without entanglement" is from 1630s. Meaning "to leap clear over" is first attested 1791. Meaning "get approval for" (a proposal, etc.) is from 1944; meaning "establish as suitable for national security work" is from 1948. Related: Cleared; clearing.

To clear (one's) throat is from 1881; earlier clear (one's) voice (1701). To clear out "depart, leave" (1825), perhaps is from the notion of ships satisfying customs, harbor regulations, etc., then setting sail. To clear up is from 1620s, of weather; 1690s as "make clear to the mind." Clear the decks is what is done on a ship before it moves.
clear (adv.): "quite, entirely, wholly," c. 1300, from clear (adj.).
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.): late 14c., "action of making clear," verbal noun from clear (v.). Meaning "land cleared of wood" is from 1818, American English.
clearing-house (n.): also clearinghouse, 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" (source also of Middle Low German klot, klute, Middle Dutch cloot, Dutch kloot, Old High German kloz, German kloß "clod, dumpling"). In Middle English, a wedge of wood bolted to a spar, etc., to keep it from slipping (late 14c.). 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, 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;" traditionally first used in this sense by U.S. publicist Joseph I. Breen (1888-1965), head of the Production Code Administration (replaced 1945 by Eric Johnston), enforcers of Hollywood self-censorship, in reference to Jane Russell's costumes and poses in "The Outlaw."
cleave (v.1): "to split," Old English cleofan, cleven, cliven "to split, separate" (class II strong verb, past tense cleaf, past participle clofen), from Proto-Germanic *kleuban (source also of 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," Middle English cleven, clevien, cliven, from Old English clifian, cleofian, from West Germanic *klibajan (source also of Old Saxon klibon, Old High German kliban, Dutch kleven, Old High German kleben, German kleben "to stick, cling, adhere"), 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 part of a surname from mid-14c. Meaning "butcher's chopper" is from mid-15c.
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 (adj.): also cleche, "pierced through with a figure of the same kind," 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," 1885, from Latinized form of Greek kledon "omen or presage contained in a word or sound," also "report, rumor, tidings; fame" (see claim (v.)) + -ism.
clef (n.): 1570s in a musical sense, "character on a staff to indicate its name and pitch," 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, violin, or G-clef, which crosses on the second line of the staff, denoting that as the G above middle C on the piano.
cleft (n.): 1570s, alteration (by influence of cleft, new weak past participle of cleave (v.1)), of Middle English clift (early 14c.), from Old English geclyft (adj.) "split, cloven," from Proto-Germanic *kluftis (source also of Old High German and German kluft, Danish kløft "cleft"), from PIE *gleubh- (see glyph). In Middle English anatomy, it meant "the parting of the thighs" (early 14c.).
cleft (adj.): late 14c., past participle adjective from cleave (v.1)). Cleft palate attested from 1828.
Clematis (n.): plant genus, 1550s, "periwinkle," from Latin Clematis, from Greek klematis, in Dioscorides as the name of a climbing or trailing plant (OED says probably the periwinkle) with long and lithe branches, 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, compare inclined in secondary meaning "disposed favorably." Earlier in same sense was clemence (late 15c.).
Clemency is exercised only toward offenders, being especially the attribute of those in exalted places having power to remit or lighten penalty. [Century Dictionary]
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.
Clementine (adj.): 1705, in reference to various popes who took the name Clement (see clement (adj.)), especially of the edition of the Vulgate issued due to Pope Clement V (1309-14).
Clementine: fem. proper name, from fem. of Clement (see clement (adj.)).
clench (v.): Old English (be)clencan "to hold fast, make cling," causative of clingan (see cling); compare stench/stink. Related: Clenched; clenching.
clench (n.): "part of a nail that clinchers," 1590s, from clench (v.). Meaning "a grasp, grip" is from 1779.
Cleon: masc. proper name, from Latinized form of Greek kleon, kleos "fame" (see Clio).
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" (see clavicle) + patris, genitive of pater "father" (see father (n.)). The famous queen was the seventh of that name.
clepe (v.): "to call; to name" (archaic), from Old English cleopian, clipian "to speak, call; summon, invoke; implore."
clepsydra (n.): "ancient Greek water clock," 1640s, from Latinized form of Greek klepsydra, from stem of kleptein "to steal, to hide" (see kleptomania) + hydor "water" (see water (n.1)).
cleptomaniac (n.): Latinized variant of kleptomaniac. Related: Cleptomania.
clerestory (n.): early 15c., probably from clere "clear," in a sense "light, lighted" (see clear (adj.)), and story (n.2), 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; so called because pierced with windows. Related: Clerestorial.
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 (n.)); 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 (n.): 1670s, a nun, from clergy + man (n.). Not seriously as "woman pastor" until 1871; in between it was used humorously for "old woman" or "domineering wife of a clergyman." Compare clergyman.
cleric (n.): 1620s (also in early use as an adjective), from Church Latin clericus "clergyman, priest," noun use of adjective meaning "priestly, belonging to the clerus;" from Ecclesiastical Greek klerikos "pertaining to an inheritance," but in Greek Christian jargon by 2c., "of the clergy, belonging to the clergy," as opposed to the laity; from kleros "a lot, allotment; piece of land; heritage, inheritance," originally "a shard or wood chip used in casting lots," related to klan "to break" (see clastic).

Kleros was 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." Or else it is from the use of the word in Acts i:17. A word taken up in English after clerk (n.) shifted to its modern meaning.
clerical (adj.): 1590s, "pertaining to the clergy," from cleric + -al (1), or from French clérical, from Old French clerigal "learned," from Latin clericalis, from clericus (see cleric). 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.
clerisy (n.): 1818, on model of German clerisei, from Late Latin clericia, related to clericus (see cleric); coined by Coleridge "to express a notion no longer associated with CLERGY" [OED].
clerk (n.): "man ordained in the ministry," c. 1200, from Old English cleric and Old French clerc "clergyman, priest; scholar, student," both from Church Latin clericus "a priest," noun use of adjective meaning "priestly, belonging to the clerus" (see cleric).

Modern bureaucratic usage is a reminder of the dark ages when clergy alone could read and write and were employed for that skill by secular authorities. In late Old English the word can mean "king's scribe; keeper of accounts;" by c. 1200 clerk took on a secondary sense in Middle English (as the cognate word did in Old French) of "anyone who can read or write." This led to the sense "assistant in a business" (c. 1500), originally a keeper of accounts, later, especially in American English, "a retail salesman" (1790). Related: Clerkship.
clerk (v.): "act as a clerk," 1550s, from clerk (n.). Related: Clerked, clerking.
cleromancy (n.): "divination by dice," c. 1600, from French cléromancie, from Latinized form of Greek kleros "lot" (see clerk (n.)) + manteia "oracle, divination" (see -mancy).
Cleveland: city in Ohio, U.S., laid out 1796 by Gen. Moses Cleaveland (1754-1806) and later named for him. His descendants included U.S. President Grover Cleveland (1837-1908). The family name is from place names in England based on Middle English cleove, a variant of cliff.
clever (adj.): 1580s, "handy, dexterous," apparently from East Anglian dialectal cliver "expert at seizing," perhaps from East Frisian klüfer "skillful," or Norwegian dialectic klover "ready, skillful," and perhaps influenced by Old English clifer "claw, hand" (early usages seem to refer to dexterity). Or perhaps akin to Old Norse kleyfr "easy to split" and from a root related to cleave "to split." Extension to intellect is first recorded 1704.
This is a low word, scarcely ever used but in burlesque or conversation; and applied to any thing a man likes, without a settled meaning. [Johnson, 1755]
The meaning has narrowed since, but clever also often in old use and dialect meant "well-shaped, attractive-looking" and in 19c. American English sometimes "good-natured, agreeable." Related: Cleverly; cleverness.
clevis (n.): "U-shaped iron bar with holes for a bolt or pin, used as a fastener," 1590s, of unknown origin, perhaps from the root of cleave (v.2). Also uncertain is whether it is originally a plural or a singular.
clew (n.): "ball of thread or yarn," northern English and Scottish relic of Old English cliewen "sphere, ball, skein, ball of thread or yarn," probably from West Germanic *kleuwin (source also of Old Saxon cleuwin, Dutch kluwen), from Proto-Germanic *kliwjo-, from PIE *gleu- "gather into a mass, conglomerate" (see clay).
cliche (n.): 1825, "electrotype, stereotype," from French cliché, a technical word in printer's jargon for "stereotype block," noun use of past participle of clicher "to click" (18c.), supposedly echoic of the sound of a mold striking molten metal. Figurative extension to "trite phrase, worn-out expression" is first attested 1888, following the course of stereotype. Related: Cliched (1928).
click (v.): 1580s, of imitative origin (compare Dutch and East Frisian klikken "to click; Old French clique "tick of a clock"). The figurative sense, in reference usually to persons, "hit it off at once, become friendly upon meeting" is from 1915, perhaps based on the sound of a key in a lock. Related: Clicked; clicking.
click (n.): 1610s, from click (v.). Click-beetle attested from 1830.
click-bait (n.): Internet content meant primarily to lure a viewer to click on it, by 2011, from click (n.) + bait (n.).