Online Etymology Dictionary

classic (n.): "a Greek or Roman writer or work," 1711, from classic (adj.). So, by mid-19c., any work in any context held to have a similar quality or relationship. In classical Latin noun use of classicus meant "a Marine" (miles classicus) from the "military division" sense of classis.
classical (adj.): 1590s, "of the highest rank" (originally in literature), from classic + -al (1). Classical music (1836) was defined originally against romantic music.
[I]n general, as now used, the term classical includes the composers active in instrumental music from somewhere about 1700 to say 1830. Hence the list includes among the great names those of Bach, his sons, Haydn, Mozart, Beethoven, Clementi, Dussek, Pleyel, Cramer, etc. The next step beyond the term classical is "modern romantic," the composers of which school may be taken to include all the writers for pianoforte from about 1829 (when Mendelssohn published the first "Songs without Words") down to the present. The term romantic in this sense means strongly marked, extraordinary, intending to tell stories and the like. ["Music, Its Ideals and Methods," W.S.B. Mathews, 1897]
But already by 1880s it was acknowledged the term had a double sense: Music that had withstood the test of time, as well as music of a style contrasted to "romantic." Later (early 20c.) it was contrasted to jazz (in this sense more often with reference to the orchestras than to the music itself). Still later in contrast to popular music generally (mid-20c.). Classical history is the history of ancient Greece and Rome; ancient history is the history of mankind from the earliest reliable records to the fall of Rome (476 C.E.).
classicism (n.): "classical style in art or literature," 1830, from classic + -ism. Related: Classicist.
classics (n.): "Greek and Roman writers and works," 1711, from classic (adj.).
classifiable (adj.): 1820, from classify + -able.
classification (n.): 1772, "action of classifying," noun of action from Latin stem of classify, or from French classification. Meaning "result of classifying" is from 1789.
classificatory (adj.): 1825, from Latin stem of classify + -ory.
classified (adj.): "arranged in classes," 1828, past participle adjective from classify. Meaning "secret" (of government information) is from 1941, American English. Classifieds (n.) "newspaper advertisements arranged by classes," 1913, is short for classified advertisements
classify (v.): 1782, from French classifier, from classe (see class (n.)) + -fier (see -fy). Related: Classified; classifying.
classism (n.): "distinction of class," 1842, from class (n.) + -ism.
classless (adj.): 1874 in the social sense (1863 in reference to class generally), from class (n.) in the "social order" sense + -less. As "lacking the sophistication of high class," by 1979. Related: Classlessly; classlessness.
classmate (n.): "one of the same class at school or college," 1713, from class (n.) + mate (n.).
classroom (n.): also class-room, 1811, from class (n.) + room (n.).
classy (adj.): "pertaining to or characteristic of a (high) class," 1891, from class (n.) + -y (2). Related: Classily; classiness.
clastic (adj.): "consisting of broken pieces," 1875, in geology, from Latinized form of Greek klastos "broken in pieces," from klan, klaein "to break," from PIE *kla-, variant of root *kel- (1) "to strike" (see holt).
clatter (v.): late Old English clatrung "clattering, noise," verbal noun implying an Old English *clatrian, of imitative origin. Compare Middle Dutch klateren, East Frisian klatern, dialectal German klattern. The noun is attested from mid-14c.
Claude: masc. proper name, from French form of Claudius.
Claudia: fem. proper name, from Latin Claudius (m.).
claudication (n.): 1550s, from Middle French claudication (13c.) or directly from Latin claudicationem (nominative claudicatio) "a limping," noun of action from past participle stem of claudicare "to limp, be lame," from claudus "limping, halting, lame." Related: Claudicant (adj.); claudicate.
Claudius: masc. proper name, from the name of two Roman gentes, perhaps related to claudus "lame," which is of unknown origin. Related: Claudian.
clausal (adj.): 1870, from clause + -al (1).
clause (n.): c. 1200, "a sentence, a brief statement, a short passage," from Old French clause "stipulation" (in a legal document), 12c., from Medieval Latin clausa "conclusion," used in the sense of classical Latin clausula "the end, a closing, termination," also "end of a sentence or a legal argument," from clausa, fem. noun from past participle of claudere "to close, to shut, to conclude" (see close (v.)). Grammatical sense is from c. 1300. Legal meaning "distinct condition, stipulation, or proviso" is recorded from late 14c. in English. The sense of "ending" seems to have fallen from the word between Latin and French.
claustral (adj.): "resembling a cloister," early 15c., from Middle French claustral (15c.) and directly from Medieval Latin claustralis "pertaining to a claustrum" (see cloister).
claustration (n.): 1863, "act of shutting up in a cloister," as if from a noun of action formed in Latin from Latin claustrare, from claustrum (see cloister).
claustrophilia (n.): "morbid desire to be shut up in a confined space," 1884, from claustro-, abstracted from claustrophobia, + -philia.
claustrophobia (n.): "morbid fear of being shut up in a confined space," coined 1879 (in article by Italian-born, French-naturalized Swiss-English physician Dr. Benjamin Ball (1834-1892)) from Latin claustrum "a bolt, a means of closing; a place shut in, confined place, frontier fortress" (in Medieval Latin "cloister"), 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.
claves (n.): pair of hardwood sticks used in making music, 1928, from American Spanish claves (plural), from Spanish clave "keystone," from Latin clavis "key" (see slot (n.2)).
clavichord (n.): mid-15c., from Medieval Latin clavicordium (15c.), from Latin clavis "a key" (see slot (n.2)) + chorda "a string" (see cord).
clavicle (n.): "collarbone," 1610s, from Middle French clavicule "collarbone" (16c.), also "small key," from Medieval Latin clavicula "collarbone" (used c.980 in a translation of Avicenna), special use of classical Latin clavicula, literally "small key, bolt," diminutive of clavis "key" (see slot (n.2)); in the anatomical sense 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, Danish klaver, etc. The German word is the direct source of the name of the musical instrument, a sense attested from 1845 in English.
claviger (n.): "one who carries a club," c. 1600, from Latin claviger (an epithet of Hercules), from clava "club, knotty branch" + stem of gerere "to bear" (see gest).
claw (n.): Old English clawu, earlier clea, "claw, talon, iron hook," from Proto-Germanic *klawo (source also of Old Frisian klawe "claw, hoe," Middle Dutch klouwe, Dutch klauw, Old High German klawa, German Klaue "claw"). Claw-foot in reference to furniture is from 1823; claw-and-ball attested from 1893. Claw-hammer attested from 1769.
claw (v.): Old English clawian "to scratch, claw," from the same root as claw (n.). Related: Clawed; clawing. Compare Dutch klaauwen, Old High German klawan, German klauen. To claw back"regain by great effort" is from 1953; as a noun, an act of this, from 1969.
clay (n.): Old English clæg "stiff, sticky earth; clay," from Proto-Germanic *klaijaz (source also of Old High German kliwa "bran," German Kleie, Old Frisian klai "clay," Old Saxon klei, Middle Dutch clei, Danish klæg "clay;" also Old English clæman, Old Norse kleima, Old High German kleiman "to cover with clay"), from PIE root *glei- "clay" (source also of Greek gloios "sticky matter;" Latin gluten "glue;" Old Church Slavonic glina "clay," glenu "slime, mucus;" Old Irish glenim "I cleave, adhere").

in Scripture, the stuff from which the body of the first man was formed; hence "human body" (especially when dead). Clay pigeon is from 1888. Feet of clay "fundamental weakness" is from Dan. ii:33.
clayey (adj.): Old English clæig, from contracted compound of clæg (see clay) + -ig (see -y (2)).
claymore (n.): 1749, "two-edged broadsword of ancient Scottish Highlanders," from Gaelic claidheamh mor "great sword," from claidheb "sword" (compare Welsh cleddyf), possibly from PIE root *kel- (1) "to strike" (see holt) + mor "great" (compare Welsh mawr; 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 "free from dirt or filth; pure, chaste, innocent; open, in the open," of beasts, "ritually safe to eat," from West Germanic *klainoz "clear, pure" (source also of 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- "bright, gleaming" (source also of Greek glene "eyeball," Old Irish gel "bright").

"Largely replaced by clear, pure in the higher senses" [Weekley], but as a verb (mid-15c.) it has largely usurped what once belonged to cleanse. Meaning "whole, entire" is from c. 1300 (clean sweep in the figurative sense is from 1821). Sense of "innocent" is from c. 1300; that of "not lewd" is from 1867; that of "not carrying anything forbidden" is from 1938; that of "free of drug addiction" is from 1950s. To come clean "confess" is from 1919, American English.
clean (v.): mid-15c., "make clean," from clean (adj.). Related: Cleaned; cleaning. From clean out "clean by emptying" comes sense of "to leave bare" (1844); cleaned-out "left penniless by losses" is from 1812.
clean (adv.): Old English clæne "dirtlessly," also "clearly, fully, entirely;" see clean (adj.). Compare similar use of German rein "clean."
clean-cut (adj.): "precise," 1829 (of turf), from clean (adv.) + past participle of cut (v.).
clean-living (adj.): 1920, from clean (adv.) + live (v.).
clean-up (n.): also cleanup, 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.
cleaner (n.): mid-15c., agent noun from clean (v.). Meaning "shop that cleans clothes" is from 1873. 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 "morally pure, innocent," from clæne (see clean (adj.)). Of persons, "habitually clean," from c. 1500.
cleanly (adv.): Old English clænlice; see clean (adj.) + -ly (2).
cleanness (n.): Old English clænnes "(moral) cleanness, purity, chastity;" see clean (adj.) + -ness. Meaning "absence of dirt or filth" is late 14c.
cleanse (v.): Old English clænsian "to cleanse, purge, purify, chasten, justify," from West Germanic *klainson, from *klainoz (see clean (adj.)). Despite its modern spelling (16c.), it retains its Middle English pronunciation. Related: Cleansed; cleansing.
cleanser (n.): 1520s, "a purgative;" 1560s, "one who cleans," agent noun from cleanse (v.). Meaning "thing that cleanses" is from late 14c. Old English had clænsere "priest."