Online Etymology Dictionary

click (n.): 1610s, from click (v.). Click-beetle attested from 1830.
client (n.): late 14c., from Anglo-French clyent (c.1300), from Latin clientem (nominative cliens) "follower, retainer," perhaps a variant of present participle of cluere "listen, follow, obey" (see listen); or, more likely, from clinare "to incline, bend," from suffixed form of PIE root *klei- "to lean" (see lean (v.)).

The ground sense apparently is of one who leans on another for protection. In ancient Rome, a plebian under protection of a patrician (called patronus in this relationship; see patron); in English originally "a lawyer's customer," by c.1600 extended to any customer.
clientele (n.): 1560s, "body of professed adherents," from French clientèle (16c.), from Latin clientela "relationship between dependent and patron, body of clients," from clientem (nominative cliens; see client). Meaning "customers, those who regularly patronize a business or professional" is from 1865, perhaps a reborrowing from French.
cliff (n.): Old English clif "rock, promontory, steep slope," from Proto-Germanic *kliban (cf. Old Saxon clif, Old Norse klif, Middle Dutch klippe, Dutch klip, Old High German klep, German Klippe "cliff, promontory, steep rock").

Clift has been a variant spelling since 15c. and was common in early Modern English, influenced by or merged with clift, a variant of cleft (n.). Cliff-dweller first attested 1889, American English.
cliff-hanger (n.): also cliffhanger, "suspenseful situation," 1937, in reference to U.S. cinema serials, agent noun from cliff + agent noun from hang (v.). In some cases, especially Westerns, the hero or heroine literally was dangling from a cliff at the end of an episode.
climacteric: c.1600 (adj.), 1620s (n.), from Latin climactericus, from Greek klimakterikos "of a critical period," from klimakter "rung of a ladder" (see climax (n.)). A critical stage in human life, a period supposed to be especially liable to change. By some, held to be the years that are multiples of 7 (7, 14, 21, etc.), by others only the odd multiples (7, 21, 35, etc.), and by still others the multiples of 9. The Great Climacteric was the 63rd year (7x9), supposed to be especially critical.
climactic (adj.): "pertaining to a climax," 1832, from climax, apparently on the analogy of syntax/syntactic. Related: Climactical.
climate (n.): late 14c., "horizontal zone of the earth," Scottish, from Old French climat "region, part of the earth," from Latin clima (genitive climatis) "region; slope of the Earth," from Greek klima "region, zone," literally "an inclination, slope," thus "slope of the Earth from equator to pole," from root of klinein "to slope, to lean" (see lean (v.)).

The angle of sun on the slope of the Earth's surface defined the zones assigned by early geographers. Early references in English, however, are in astrology works, as each of the seven (then) climates was held to be under the influence of one of the planets. Shift from "region" to "weather associated with a region" perhaps began in Middle English, certainly by c.1600.
climate change (n.): 1983, in the modern "global warming" sense.
climatic (adj.): 1828, from climate + -ic. There is a 1650 citation for climatical in OED. Related: Climatically.
climatography (n.): 1813, from comb. form of climate + -graphy. Related: Climatographic.
climatological (adj.): 1827, from climatology + -ical. Related: Climatologically.
climatologist (n.): 1844, from climatology + -ist.
climatology (n.): "scientific study of climates," 1803, from climate + -ology.
climax (n.): 1580s, in the rhetorical sense (a chain of reasoning in graduating steps from weaker to stronger), from Late Latin climax (genitive climacis), from Greek klimax "propositions rising in effectiveness," literally "ladder," from root of klinein "to slope," from PIE root *klei- "to lean" (see lean (v.)).

The rhetorical meaning evolved in English through "series of steps by which a goal is achieved," to "escalating steps," to (1789) "high point of intensity or development," a usage credited by the OED to "popular ignorance." The meaning "sexual orgasm" is recorded by 1880 (also in terms such as climax of orgasm), said to have been promoted from c.1900 by birth-control pioneer Marie Stopes (1880-1958) and others as a more accessible word than orgasm (n.).
climax (v.): 1835, "to reach the highest point," from climax (n.). Related: Climaxed; climaxing.
climb (v.): Old English climban "raise oneself using hands and feet; rise gradually, ascend; make an ascent of" (past tense clamb, past participle clumben, clumbe), from West Germanic *klimbanan "go up by clinging" (cf. Dutch klimmen "to climb," Old High German klimban, German klimmen). A strong verb in Old English, weak by 16c. Most other Germanic languages long ago dropped the -b. Meaning "to mount as if by climbing" is from mid-14c. Figurative sense of "rise slowly by effort" is from mid-13c. Related: Climbed; climbing.
climb (n.): 1580s, "act of climbing," from climb (v.). Meaning "an ascent by climbing" is from 1915, originally in aviation.
climbable (adj.): 1610s, from climb (v.) + -able.
climber (n.): early 15c., "one who climbs," agent noun from climb (v.). Of plants, from 1630s.
clime (n.): 1540s, shortening of climate (or a nativization of Latin clima). It might usefully take up the old, abandoned "horizontal region of the earth" sense of climate, but it is used chiefly by the poets and with no evident agreement on just what they mean by it.
clinch (v.): 1560s, "clasp, interlock," especially with a bent nail, variant of clench. The sense of "settle decisively" is first recorded 1716, from the notion of "clinching" the point of a nail to keep it fast. Boxing sense is from 1860. Related: Clinched; clinching.
clinch (n.): 1620s, "method of fastening," from clinch (v.). Meaning "a fastening by bent nail" is from 1650s. In pugilism, from 1875.
clincher (n.): early 14c., "person or thing that clinches" (i.e., secures nails by bending down or riveting the pointed end), late 15c. as a class of shipyard worker; agent noun from clinch (v.). As a type of nail, from 1735; as a conclusive statement, argument, etc., 1737.
cline (n.): 1938, in biological use, back-formation from incline or from Greek klinein "to slope, to lean" (see lean (v.)). Middle English had clinen (v.) "to bend, bow," from Old French cliner, from Latin clinare.
cling (v.): Old English clingan "hold fast, adhere closely; congeal, shrivel" (strong verb, past tense clang, past participle clungen), from Proto-Germanic *klingg- (cf. Danish klynge "to cluster;" Old High German klinga "narrow gorge;" Old Norse klengjask "press onward;" Danish klinke, Dutch klinken "to clench;" German Klinke "latch").

The main sense shifted in Middle English to "adhere to" (something else), "stick together." Of persons in embrace, c.1600. Figuratively (to hopes, outmoded ideas, etc.), from 1580s. Of clothes from 1792. Related: Clung; clinging.
clingstone (n.): "fruit having the pulp adhering firmly to the stone," 1722, from cling (v.) + stone (n.). Also as an adjective.
clingy (adj.): 1680s, of things, from cling + -y (2). Of persons (especially children) from 1969, though the image of a "clingy vine" in a relationship goes back to 1896. Related: Clinginess.
clinic (n.): 1620s, from French clinique (17c.), from Latin clinicus "physician that visits patients in their beds," from Greek klinike (techne) "(practice) at the sickbed," from klinikos "of the bed," from kline "bed, couch, that on which one lies," from suffixed form of PIE root *kli- "lean, slope" (see lean (v.)).

Originally in English "bedridden person;" sense of "hospital" is 1884, from German Klinik in this sense, itself from French clinique, via the notion of "bedside medical education." The modern sense is thus reversed from the classical, when the "clinic" came to the patient. General sense of "conference for group instruction in something" is from 1919.
clinical (adj.): 1780, "pertaining to hospital patients or hospital care," from clinic + -al (2). Meaning "coldly dispassionate" (like a medical report) is recorded from 1928. Related: Clinically.
clinician (n.): 1844, from French clinicien, from Latin clinicus (see clinic).
clink (v.): early 14c., echoic (cf. Dutch klinken, Old High German klingan, German klingen). Related: Clinked; clinking. The noun in the sound sense is from c.1400.
clink (n.2): "prison," 1770s, apparently originally (early 16c.) the Clynke on Clink Street in Southwark, on the estate of the bishops of Winchester. To kiss the clink "to be imprisoned" is from 1580s, and the word and the prison name might be cognate derivatives of the sound made by chains or metal locks (see clink (v.)).
clink (n.1): "sharp, ringing sound made by collision of sonorous (especially metallic) bodies," c.1400, from clink (v.).
clinker (n.): "mass of slag," 1769, from klincard (1640s), a type of paving brick made in Holland, from Dutch klinkaerd, from klinken "to ring" (as it does when struck), of imitative origin. Also "a clinch-nail;" hence clinker-built (1769). The meaning "stupid mistake" is first recorded 1950 in American English; originally (1942) "a wrong note in music."
clino-: before vowels clin-, word-forming element meaning "slope, slant, incline," from Latinized comb. form of Greek klinein "to lean, slope" (see lean (v.)).
clinometer (n.): "measurer of slopes and elevations," 1811, from clino- + -meter. Related: Clinometric.
Clio: "muse of history, muse who sings of glorious actions," usually represented with a scroll and manuscript case, from Latin Clio, from Greek Kleio, literally "the proclaimer," from kleiein "to tell of, celebrate, make famous," from kleos "rumor, report, news; good report, fame, glory," from PIE *klew-yo-, from root *kleu- "to hear" (see listen). Related to the -kles in Damocles, etc.
clip (v.1): "to cut or sever with a sharp instrument," c.1200, from a Scandinavian source (cf. Old Norse klippa, Swedish klippa, Danish klippe "clip, shear, cut") probably echoic. Related: Clipped; clipping.

Meaning "to pronounce short" is from 1520s. The verb has a long association with shady activities, originally especially in reference to cutting or shaving metal from coins (c.1400), but later extended to swindles from the sense "to shear sheep," hence clip-joint "place that overcharges outrageously" (1933, American English, a term from Prohibition). To clip (someone's) wings figuratively (1590s) is from the method of preventing a captive bird from flying.
clip (v.2): "fasten, hold together by pressure," also (mostly archaic) "to embrace," from Old English clyppan "to embrace, clasp; surround; prize, honor, cherish;" related to Old Frisian kleppa "to embrace, love," Old High German klaftra, German klafter "fathom" (on notion of outstretched arms). Also cf. Lithuanian glebys "armful," globiu "to embrace, support." Meaning "to fasten, bind" is early 14c. Meaning "to fasten with clips" is from 1902. Related: Clipped; clipping. Original sense of the verb is preserved in U.S. football clipping penalty.
clip (n.1): "something for attaching or holding," mid-14c., probably from clip (v.2). Meaning "receptacle containing several cartridges for a repeating firearm" is from 1901. Meaning "piece of jewelry fastened by a clip" is from 1937. This is also the source of paper clip (1854). Old English had clypp "an embrace."
clip (n.2): mid-15c., "shears," from clip (v.1). Meaning "act of clipping" is from 1825, originally of sheep-shearing, later of haircuts. Meaning "rate of speed" is 1867 (cf. clipper). Meaning "an extract from a movie" is from 1958.
clip-clop: sound as of a horse's hooves, 1884, imitative.
clip-on (adj.): 1909, from clip (v.2) + on.
clipboard (n.): 1904, from clip (n.1) + board (n.1). Portable board with a hinged clip at the top to hold papers.
clipper (n.): late 14c., "sheepshearer;" early 15c., "a barber;" c.1300 as a surname; agent noun from Middle English clippen "shorten" (see clip (v.1)). The type of fast sailing ship so called from 1823 (in Cooper's "The Pilot"), probably from clip (v.1) in sense of "to move or run rapidly," hence early 19c. sense "person or animal who looks capable of fast running." Perhaps originally simply "fast ship," regardless of type:
Well, you know, the Go-along-Gee was one o' your flash Irish cruisers -- the first o' your fir-built frigates -- and a clipper she was! Give her a foot o' the sheet, and she'd go like a witch--but somehow o'nother, she'd bag on a bowline to leeward. ["Naval Sketch-Book," by "An officer of rank," London, 1826]
The early association of the ships was with Baltimore, Maryland. Perhaps influenced by Middle Dutch klepper "swift horse," echoic (Clipper appears as the name of an English race horse in 1831). In late 18c., the word principally meant "one who cuts off the edges of coins" for the precious metal.
clippers (n.): "shears-like cutting tool for hair, etc.," 1876, agent noun from clip (v.1). Earlier they were clipping shears (mid-15c.).
clipping (n.1): early 13c., "clasping, embracing," verbal noun from clip (v.2). As a U.S. football penalty (not in OED), from 1920.
Clipping or Cutting Down from Behind. -- This is to be ruled under unnecessary roughness, and penalized when it is practiced upon "a man obviously out of the play." This "clipping" is a tendency in the game that the committee is watching anxiously and with some fear. ["Colliers," April 10, 1920]
clipping (n.2): "a cutting," early 14c., verbal noun from clip (v.1). Sense of "a small piece cut off" is from late 15c. Meaning "an article cut from a newspaper" is from 1857.
clique (n.): 1711, "a party of persons; a small set, especially one associating for exclusivity," from obsolete French clique, originally (14c.) "a sharp noise," also "latch, bolt of a door," from Old French cliquer "click, clatter, crackle, clink," 13c., echoic. Apparently this word was at one time treated in French as the equivalent of claque (q.v.) and partook of that word's theatrical sense.