Online Etymology Dictionary

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.
clerk (v.): 1550s, from clerk (n.). Related: Clerked, clerking.
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.): late 16c., "handy, dexterous," from East Anglian dialectal cliver "expert at seizing," perhaps from East Frisian klufer or Norwegian dialectic klover "ready, skillful," and perhaps influenced by Old English clifer "claw, hand" (early usages seem to refer to dexterity); 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]
clevis (n.): "U-shaped piece of iron 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," probably from West Germanic *kleuwin (cf. Old Saxon cleuwin, Dutch kluwen), from Proto-Germanic *kliwjo-, from PIE *gleu- "gather into a mass, conglomerate" (see clay).
cliche (n.): 1832, from French cliché, a technical word in printer's jargon for "stereotype," supposedly echoic of the sound of a mold striking molten metal, thus past participle of clicher "to click" (18c.). Figurative extension to "worn-out expression" is first attested 1888, following the course of stereotype. Related: Cliched (1928).
click (v.): 1580s, of imitative origin (cf. Dutch and East Frisian klikken "to click; Old French clique "tick of a clock"). The figurative sense of "fit together" (based on the sound of a key in a lock, etc.) first recorded 1915. Related: Clicked; clicking.
click (n.): 1610s, from click (v.).
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 is of one who leans on another for protection. In ancient Rome, a plebian under protection of a patrician (in this relationship called patronus, see patron); originally in English "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" is from 1865, perhaps a reborrowing from French.
cliff (n.): Old English clif, from Proto-Germanic *kliban (cf. Old Saxon clif, Old Norse klif, Old High German klep, Middle Dutch klippe, German Klippe "cliff, steep rock"), perhaps from PIE root *gleibh- "to adhere, be attached." Cliff-dweller first attested 1889, American English.
cliffhanger (n.): "suspenseful situation," 1937, in reference to U.S. cinema serials, agent noun from cliff + hang. 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.): 1872, from climax, apparently on the analogy of syntax/syntactic.
climate (n.): late 14c., Scottish, from Old French climat, from Latin clima (genitive climatis) "region, slope of the Earth," from Greek klima "region, zone," from root of klinein "to slope," thus "slope of the Earth from equator to pole," from PIE root *klei- "to lean" (see lean (v.)). The angle of sun on the slope of the Earth's surface defined the zones assigned by early geographers. Meaning moved from "region" to "weather associated with a region" by c.1600. Related: Climatography.
climate change (n.): 1983, in the modern "global warming" sense.
climatic (adj.): 1828, from climate + -ic.
climatology (n.): 1803, from climate + -ology. Related: Climatologist (1886).
climax (n.): 1580s, in the rhetorical sense, 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," 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.
climax (v.): 1835, from the noun. Related: Climaxed; climaxing.
climb (v.): Old English climban, from West Germanic *klimbanan "go up by clinging" (cf. Old High German klimban, German klimmen). A strong verb in Old English, weak by 16c. Most other Germanic languages long ago dropped the -b. Related: Climbed; climbing.
climb (n.): late 16c., from climb (v.).
climber (n.): early 15c., "one who climbs," agent noun from climb (v.). Of plants, from 1630s.
clime (n.): 1540s, chiefly poetic shortening of climate.
clinch (v.): 1560s, "clasp, interlock," 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. The noun is from 1620s.
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" (see lean (v.)).
cling (v.): Old English clingan "hold fast, congeal, shrivel," 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 by 13c. to "adhere, stick together." Related: Clinging.
clingy (adj.): c.1710, 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.
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.)). An adjective originally in English, then "sick person;" sense of "hospital" is 1884, from German Klinik, itself from French clinique. The modern sense is thus reversed from the classical, when the "clinic" came to the patient.
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.): 1875, from French clinicien, from Latin clinicus (see clinic).
clink (v.): late 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.): "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.)).
clinker (n.): 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. The meaning "stupid mistake" is first recorded 1950 in American English; originally (1942) "a wrong note in music."
clinometer (n.): "measurer of slopes and elevations," 1811, from clino-, element meaning "slope, slant, incline," Latinized comb. form of Greek klinein (see lean (v.)) + -meter.
Clio: muse of history, 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.