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co-[co- 词源字典]: in Latin, the form of com- in compounds with stems beginning in vowels and h- and gn- (see com-). Taken in English from 17c. as a living prefix meaning "together, mutually, in common," and used promiscuously with native words and Latin-derived words not beginning with vowels, sometimes even with words already having it (such as co-conspiritor).[co- etymology, co- origin, 英语词源]
co-ed (n.): also coed, 1886, American English, (first in Louisa Mae Alcott's "Jo's Boys"); short for "co-educational system;" 1889 as an adjective, short for coeducational; 1893 as a noun meaning "girl or woman student at a co-educational institution."
co-op (n.): 1861, abbreviation of cooperative. The hyphen is needed to avoid confusion with coop (n.).
co-opt (v.): 1650s, "to select (someone) for a group or club by a vote of members," from Latin cooptare "to elect, to choose as a colleague or member of one's tribe," from com- "together" (see com-) + optare "choose" (see option (n.)). For some reason this defied the usual pattern of Latin-to-English adaptation, which should have yielded *cooptate. Sense of "take over" is first recorded c. 1953. Related: Co-opted; co-opting.
co-ordinate: see coordinate.
co-star: also costar, 1919 as a verb; 1926 as a noun, from co- + star (v.).
co.: abbreviation of company, attested by 1670s.
coach (n.): 1550s, "large kind of carriage," from Middle French coche (16c.), from German kotsche, from Hungarian kocsi (szekér) "(carriage) of Kocs," village where it was first made. In Hungary, the thing and the name for it date from 15c., and forms are found in most European languages (Spanish and Portuguese coche, Italian cocchino, Dutch koets). Applied to railway cars 1866, American English. Sense of "economy or tourist class" is from 1949. Meaning "instructor/trainer" is c. 1830 Oxford University slang for a tutor who "carries" a student through an exam; athletic sense is 1861.
coach (v.): 1610s, "to convey in a coach," from coach (n.). Meaning "to prepare (someone) for an exam" is from 1849. Related: Coached; coaching.
coachman (n.): 1570s, from coach (n.) + man (n.).
coagulant (n.): 1770, from Latin coagulantem (nominative coagulans), present participle of coagulare (see coagulate).
coagulate (v.): early 15c., from Latin coagulatus, past participle of coagulare "to cause to curdle," from cogere "to curdle, collect" (see cogent). Earlier coagule, c. 1400, from Middle French coaguler. Related: Coagulated; coagulating.
coagulation (n.): c. 1400, from Latin coagulationem (nominative coagulatio), noun of action from past participle stem of coagulare (see coagulate).
coal (n.): Old English col "charcoal, live coal," from Proto-Germanic *kula(n) (cognates: Old Frisian kole, Middle Dutch cole, Dutch kool, Old High German chol, German Kohle, Old Norse kol), from PIE root *g(e)u-lo- "live coal" (cognates: Irish gual "coal").

Meaning "mineral consisting of fossilized carbon" is from mid-13c. First mentioned (370 B.C.E.) by Theophrastus in his treatise "On Stones" under the name lithos anthrakos (see anthrax). Traditionally good luck, coal was given as a New Year's gift in England, said to guarantee a warm hearth for the coming year. The phrase drag (or rake) over the coals was a reference to the treatment meted out to heretics by Christians. To carry coals "do dirty work," also "submit to insult" is from 1520s. To carry coals to Newcastle (c. 1600) Anglicizes Greek glauk eis Athenas "owls to Athens."
coalesce (v.): 1540s, from Latin coalescere "to unite, grow together, become one in growth," from com- "together" (see co-) + alescere "to grow up" (see adolescent). Related: Coalesced; coalescing; coalescence; coalescent.
coalition (n.): 1610s, "the growing together of parts," from French coalition (1540s), from Late Latin coalitus "fellowship," originally past participle of Latin coalescere (see coalesce). First used in a political sense 1715.
coaming (n.): 1610s, nautical, of unknown origin.
coarse (adj.): early 15c., cors "ordinary" (modern spelling is from late 16c.), probably adjectival use of noun cours (see course (n.)), originally referring to rough cloth for ordinary wear. Developed a sense of "rude" c. 1500 and "obscene" by 1711. Perhaps related, via metathesis, to French gros, which had a similar sense development. Related: Coarsely; coarseness.
coarsen (v.): 1805, from coarse + -en (2). Related: Coarsened; coarsening.
coast (n.): "margin of the land," early 14c.; earlier "rib as a part of the body" (early 12c.), from Old French coste "rib, side, flank; slope, incline;" later "coast, shore" (12c., Modern French côte), from Latin costa "a rib," perhaps related to a root word for "bone" (compare Old Church Slavonic kosti "bone," also see osseous).

Latin costa developed a secondary sense in Medieval Latin of "the shore," via notion of the "side" of the land, as well as "side of a hill," and this passed into Romanic (Italian costa "coast, side," Spanish cuesta "slope," costa "coast"), but only in the Germanic languages that borrowed it is it fully specialized in this sense (Dutch kust, Swedish kust, German Küste, Danish kyst). French also used this word for "hillside, slope," which led to verb meaning "sled downhill," first attested 1775 in American English. Expression the coast is clear (16c.) is an image of landing on a shore unguarded by enemies.