Online Etymology Dictionary

costume (v.): 1823, from costume (n.). Related: Costumed; costuming.
cosy (adj.): chiefly British form of cozy.
cot (n.1): "small bed," 1630s, from Hindi khat "couch, hammock," from Sanskrit khatva, probably from a Dravidian source (cf. Tamil kattil "bedstead").
cot (n.2): "hut, cottage;" see cote.
cotangent (n.): from co. tangent, abbreviation of complement + tangent.
cote (n.): Old English cote, fem. of cot (plural cotu) "small house, bedchamber, den" (see cottage). Applied to buildings for animals from early 15c.
coterie (n.): 1738, from French coterie "circle of acquaintances," originally in Middle French an organization of peasants holding land from a feudal lord (14c.), from cotier "tenant of a cote" (see cottage).
coterminous (adj.): 1630s, malformed in English from co- + terminous (see terminal). Latin purists prefer conterminous.
cotillion (n.): type of dance, 1766, from French cotillion (15c.), originally "petticoat," a double diminutive of Old French cote "skirt" (see coat (n.)); its application to a kind of dance arose in France and is considered obscure by some linguists, but there are lively turns in the dance that flash the petticoats.

Meaning "formal ball" is 1898, American English, short for cotillion ball. French uses -on (from Latin -onem) to reinforce Latin nouns felt to need more emphatic power (e.g. poisson from Latin piscis). It also uses -on to form diminutives, often strengthened by the insertion of -ill-, as in the case of this word.
cotquean (n.): 1540s, obsolete, "housewife of a cot," from cot (see cottage) + quean; hence "a vulgar beldam, scold" [OED]; also used contemptuously (by Shakespeare, etc.) of men seen as overly interested in housework.
cottage (n.): late 13c., from Old French cote "hut, cottage" + Anglo-French suffix -age (probably denoting "the entire property attached to a cote"). Old French cot is probably from Old Norse kot "hut," cognate of Old English cot, cote "cottage, hut," from Proto-Germanic *kutan (cf. Middle Dutch cot, Dutch kot).

Meaning "small country residence" (without suggestion of poverty or tenancy) is from 1765. Modern French cottage is a 19c. reborrowing from English. Cottage industry is attested from 1921. Cottage cheese is attested from 1831, American English, earliest in reference to Philadelphia:
There was a plate of rye-bread, and a plate of wheat, and a basket of crackers; another plate with half a dozen paltry cakes that looked as if they had been bought under the old Court House; some morsels of dried beef on two little tea-cup plates: and a small glass dish of that preparation of curds, which in vulgar language is called smear-case, but whose nom de guerre is cottage-cheese, at least that was the appellation given it by our hostess. ["Miss Leslie," "Country Lodgings," Godey's "Lady's Book," July 1831]
cotter (n.): 1640s, perhaps a shortened form of cotterel, a dialectal word for "cotter pin or bolt, bracket to hang a pot over a fire" (1560s), itself of uncertain origin.
cotton (n.): late 13c., from Old French coton (12c.), ultimately (via Provençal, Italian, or Old Spanish) from Arabic qutn, a word perhaps of Egyptian origin. Philip Miller of the Chelsea Physic Garden sent the first cotton seeds to American colony of Georgia in 1732. Also ultimately from the Arabic word, Dutch katoen, German Kattun, Provençal coton, Italian cotone, Spanish algodon, Portuguese algodão. Cotton gin is recorded from 1794 (see gin (n.2)).
cotton (v.): "to get on with" someone (usually with to), 1560s, perhaps from Welsh cytuno "consent, agree." But perhaps also a metaphor from cloth finishing and thus from cotton (n.). Related: Cottoned; cottoning.
cotton-picking (adj.): as a deprecatory term first recorded in a Bugs Bunny cartoon, but a similar noun meaning "contemptible person" dates to around 1919, perhaps with racist overtones that have faded over the years. Before mechanization, cotton picking was the most difficult labor on a cotton plantation.
I drove out to a number of the farms near Denison and found many very young white children working all day in the hot sun picking and dragging sacks of cotton. In one field the labor corps consisted of one woman and six children, one of them 5 years, one 6 years, one 7 years, one 9 years, and two about 11. The father was plowing. The 5 and 6 year olds worked all day as did the rest. The 7-year-old said he picked 50 pounds a day and the 9 year old 75 pounds. (A good picker averages several hundred a day.) School begins late on account of the cotton picking, but the children nearly all prefer school to the picking. Picking hours are long, hot, and deadly monotonous. While the very young children seem to enjoy it, very soon their distaste for it grows into all-absorbing hatred for all work. ["Field Notes of Lewis W. Hine, Child-Labor Conditions in Texas," report to U.S. Congressional Commission on Industrial Relations, 1916]
Cottonian: library in the British Museum, named for antiquarian Sir Robert Bruce Cotton (1570-1631). He donated some book to the state and his grandson donated the rest. It was badly damaged in a fire in 1731. The surname represents Old English cotum, plural of cot "cottage."
cotyledon (n.): from 1540s, in various sense, from Latin cotyledon "pennywort, navelwort," from Greek kotyledon "cup-shaped cavity," from kotyle "hollow thing, small vessel," also the name of a small liquid measure (nearly a half-pint); of uncertain origin. Botanical sense is 1776, from Linnaeus (1751).
couch (v.): c.1300, "to overlay with gold, inlay," from Old French couchier "to lay down, place; go to bed, put to bed," from Latin collocare "to lay, place, station, arrange," from com- "together" (see com-) + locare "to place" (see locate). Meaning "to put into words" is from 1520s. Related: Couched; couching. Heraldic couchant ("lying down with the head up") is late 15c., from the French present participle.
couch (n.): mid-14c., from Old French couche (12c.) "a bed, lair," from coucher "to lie down," from Latin collocare (see couch (v.)). Traditionally, a couch has the head end only raised, and only half a back; a sofa has both ends raised and a full back; a settee is like a sofa but may be without arms; an ottoman has neither back nor arms, nor has a divan, the distinctive feature of which is that it goes against a wall. Couch potato first recorded 1979.
couch-grass (n.): 1570s; the first element is a corruption of Old English cwice (see quick).
cougar (n.): 1774, from French couguar, Buffon's adaption (influenced by jaguar) of a word the Portuguese picked up in Brazil as çuçuarana, perhaps from Tupi susuarana, from suasu "deer" + rana "false." Another proposed source is Guarani guaçu ara. Evidently the cedillas dropped off the word before Buffon got it. Slang sense of "older woman (35-plus) who seeks younger males as sex partners" is attested by 2002; said in some sources to have originated in Canada, probably from some reference to predatory feline nature.
cough (v.): early 14c., coughen, probably in Old English, but not recorded, from Proto-Germanic *kokh- (with the rough "kh" of German or of Scottish loch; cf. Middle Dutch kochen, Middle High German kuchen). Onomatopoeic. Related: Coughed; coughing. As a noun from c.1300.
could (v.): Old English cuðe, past tense of cunnan "to be able" (see can (v.1)); ending changed 14c. to standard English -d(e). The excrescent -l- was added 15c.-16c. on model of would, should, where it is historical.
couldn't: by 1670s, contraction of could + not.
coulee (n.): "deep ravine, seasonally flooded," 1804, a North American word, originally in areas explored by French trappers, from French coulée "flow" (17c.), from fem. past participle of couler "to flow," from Latin colare "to filter, strain" (see colander).
coulomb (n.): 1881, named for French chemist Charles-Augustin de Coulomb (1736-1806), who devised a method of measuring electrical quantity. It is the quantity of electricity conveyed in 1 second by a current of 1 ampere. The name is a French form of Columbus.
coulrophobia (n.): "morbid fear of clowns," by 2001 (said in Web sites to date from 1990s or even 1980s), a popular term, not from psychology, possibly facetious, though the phenomenon is real enough; said to be built from Greek kolon "limb," with some supposed sense of "stilt-walker," hence "clown" + -phobia.

Ancient Greek words for "clown" were sklêro-paiktês, from paizein "to play (like a child);" or deikeliktas; other classical words used for theatrical clowns were related to "rustic," "peasant" (cf. Latin fossor "clown," literally "laborer, digger," related to fossil).

The whole creation looks suspiciously like the sort of thing idle pseudo-intellectuals invent on the Internet and which every smarty-pants takes up thereafter; perhaps it is a mangling of Modern Greek klooun "clown," which is the English word borrowed into Greek.
coulter (n.): Old English culter, from Latin culter "a knife, iron blade in a plowshare," from PIE root *(s)kel- "to cut" (see scale (n.1)). As a surname (13c.), probably from Coulter in Lancashire.
Coumadin (n.): by 1953, name for human anti-coagulant use of the rat poison warfarin sodium, abstracted from the chemical name, 3-(α-acetonylbenzyl)-4-hydroxycoumarin; earlier known as Dicoumarol, it attained publicity when it was used in 1955 to treat U.S. President Dwight Eisenhower after a heart attack. The coumarin element (1830) is from French coumarine, from coumarou, the native name in Guyana of the tonquin bean, one source of the substance.
council (n.): early 12c., from Anglo-French cuncile, from Old North French concilie (Old French concile, 12c.) "assembly; council meeting; body of counsellors," from Latin concilium "group of people, meeting," from com- "together" (see com-) + calare "to call" (see claim (v.)). Tendency to confuse it in form and meaning with counsel has been consistent since 16c.
councillor (n.): late 14c., alteration of counsellor by assimilation to council.
councilor (n.): see councillor.
counsel (n.): early 13c., from Old French counseil (10c.) "advice, counsel; deliberation, thought," from Latin consilium "plan, opinion" (see consultation). As a synonym for "lawyer," first attested late 14c.
counsel (v.): late 13c., from Old French conseiller "to advise, counsel," from Latin consiliari, from consilium "plan, opinion" (see counsel (n.)). Related: Counseled. Counseling "giving professional advice on social or psychological problems" dates from 1940.
counsellor (n.): early 13c., from Old French conseillier (Modern French conseiller), from Latin consilator, agent noun from consiliare, from consilium (see counsel (v.)). Meaning "one who gives professional legal advice" is from 1530s. Psychological sense (marriage counsellor, etc., is from 1940).
counselor (n.): see counsellor.
count (v.): mid-14c., from Old French conter "add up," but also "tell a story," from Latin computare (see compute). Related: Counted; counting. Modern French differentiates compter "to count" and conter "to tell," but they are cognates.
count (n.): title of nobility, c.1300, from Anglo-French counte (Old French conte), from Latin comitem (nominative comes) "companion, attendant," the Roman term for a provincial governor, from com- "with" (see com-) + stem of ire "to go" (see ion). The term was used in Anglo-French to render Old English eorl, but the word was never truly naturalized and mainly was used with reference to foreign titles.
countdown (n.): 1953, American English, in early use especially of launches of rockets or missiles, from count (v.) + down.
countenance (v.): late 15c., "to behave or act," from countenance (n.). Sense of "to favor, patronize" is from 1560s, from notion of "to look upon with sanction or smiles." Related: Countenanced; countenancing.
countenance (n.): mid-13c., from Old French contenance "demeanor, bearing, conduct," from Latin continentia "restraint, abstemiousness, moderation," literally "way one contains oneself," from continentem, present participle of continere (see contain). Meaning evolving Middle English from "appearance" to "facial expression betraying a state of mind," to "face" itself (late 14c.).
counter (n.): mid-14c., "table where a money lender does business," from Old French contouer, comptoir (14c.) "counting room, table of a bank," from Medieval Latin computatorium "place of accounts," from Latin computatus, past participle of computare (see compute). Generalized 19c. from banks to shops, then extended to display cases for goods. Phrase under the counter is from 1926.
counter (v.): "go against," late 14c., from Old French countre "facing opposite" (see counter-). Related: Countered; countering. As an adverb, from mid-15c.; as an adjective, from 1590s.
Counter Reformation: 1840, from counter- + Reformation.
counter-: word-forming element meaning "against; in return; corresponding," from Anglo-French countre-, French contre-, from Latin contra "opposite, contrary to, against, in return," also used as a prefix (see contra-).
counteract (v.): 1670s, from counter- + act (v.). Related: Counteracted; counteracting.
counterargument (n.): 1862, from counter- + argument.
counterattack (n.): also counter-attack, 1882, from counter- + attack (n.); as two words from early 19c. The verb is recorded from 1916.
counterbalance (v.): 1570s, from counter- + balance (v.), in reference to scales. Figurative use dates from 1630s. As a noun, from c.1600.
counterclockwise (adj., adv.): 1870, also counter-clockwise; from counter- + clockwise.