Etymology
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co. 

by 1670s as an abbreviation of company in the business sense, indicating the partners in the firm whose names do not appear in its name. Hence and co. to indicate "the rest" of any group (1757).

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co- 

in Latin, the form of com- "together, with" in compounds with stems beginning in vowels, 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 (co-worker) and Latin-derived words not beginning with vowels (codependent), including some already having it (co-conspirator).

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co-act (v.)

"to act together," c. 1600, from co- + act (v.). Related: Co-action; co-active; co-actor.

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co-education (n.)

also coeducation, "joint education," specifically of young men and young women in the same institution, 1852, from co- + education.

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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 assimilated form of 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 co-optate (which is attested from 1620s but now is rare or obsolete). Sense of "take over" is first recorded c. 1953. Related: Co-opted; co-opting.

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co-educational (adj.)

also coeducational, "involving or pertaining to joint education of men and women at the same institution," 1868, from co-education + -al (1).

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co-worker (n.)

also coworker, "one who works with another," 1640s, from co- + worker (n.). The verb co-work is attested from 1610s.

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co-respondent (n.)

in law, a joint respondent, one proceeded against along with another or others, 1857, from co- + respondent. "[S]pecifically, in Eng. law, a man charged with adultery, and made a party together with the wife to the husband's suit for divorce." [Century Dictionary].

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co-star 

also costar, 1915 as a noun; 1916 as a verb; from co- + star (v.).

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co-dependent (adj.)

also codependent, by 1905, in various senses, from co- + dependent. Modern psychological sense "dysfunctionally supporting or enabling another in a relationship in addiction or other self-destructive behavior" is attested from c. 1983. Related: Co-dependence, co-dependency.

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