coherent adj.

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).

c. 1400, from Old French hesitacion or directly from Latin haesitationem (nominative haesitatio) "a hesitation, stammering," figuratively "irresolution, uncertainty," noun of action from past participle stem of haesitare "stick fast, remain fixed; stammer in speech," figuratively "hesitate, be irresolute, be at a loss, be undecided," frequentative of haerere (past participle haesus, first person perfect indicative haesi) "to adhere, stick, cling."

This is said by Watkins to be from PIE root *ghais- "to adhere, hesitate" (source also of Lithuanian gaišti "to delay, tarry, be slow"), but some linguists reject the proposed connection; de Vaan offers no etymology.

1620s, "without coherence" (of immaterial or abstract things, especially thought or language), from in- (1) "not, opposite of" + coherent. As "without physical coherence" from 1690s. Related: Incoherently.