Kenneth G. Wilson (1923–). The Columbia Guide to Standard American English. 1993.
co- (prefix)
Three usage questions frequently arise: (1) Does co- mean “equal to” or “subordinate to” when it occurs with words such as coworker and copilot? Answer: it means either and both, and the referents and context will control. A coworker usually is neither subordinate nor superior to a fellow worker, but the copilot is always subordinate to the pilot. (2) Do newly created words using the prefix always begin as slang, or are words such as cohost and co-agent Standard from the start? Answer: such words, and nonce words using the co- prefix as well, are usually Standard almost at once, so long as the concept is clear: for example, sled-dog drivers and their codrivers are probably in the same relationship to their enterprise as pilot and copilot are to theirs: one is captain and in overall charge, and the other is the mate; and only one can actually be at the controls at any given time. (3) How should the co- prefix be attached to the base word, with a hyphen or without? Answer: usually without a space or hyphen (coincidence, coordinate), but occasionally when the base word begins with o (co-opt, co-own—but see also DIERESIS) or when a word is so new as to look odd and risk misreading without a hyphen (co-anchor or co-belligerent), a hyphen will help. The prefix co- means “together, joint,” as in co-owner; “equally, mutually,” as in coextensive, cooperative; “partner with, associate to,” as in coauthor, coproducer, or to cosign [a note]; and “assistant or subordinate to,” as in coadjutor. And sometimes the “equal or subordinate” issue is ambiguous, as in cohost, coanchor. (There are also mathematical senses meaning “complement,” as in cosine, cotangent.) Three other forms occur: col- (before l, as in collaborate); com- (before b, p, and m, as in combine, companion, and commiserate); and con- (before other sounds, as in contaminate).
