1590s, from French adhérer "to stick, adhere" (15c., corrected from earlier aderer, 14c.) or directly from Latin adhaerare "to stick, cling to," from ad "to" (see ad-) + haerere "to stick" (see hesitation). Originally often of persons, "to cleave to a leader, cause, party, etc." (compare adherent (n.), which still often retains this sense). Related: Adhered; adhering.
"to adhere, cling," Middle English cleven, clevien, cliven, from Old English clifian, cleofian "to stick fast, adhere," also figurative, from West Germanic *klibajan (source also of Old Saxon klibon, Old High German kliban, Dutch kleven, Old High German kleben, German kleben "to stick, cling, adhere"), from PIE *gloi- "to stick" (see clay).
The confusion was less in Old English when cleave (v.1) was a class 2 strong verb; but it has grown since cleave (v.1) weakened, which may be why both are largely superseded by stick (v.) and split (v.).
The main sense shifted in Middle English to "adhere to" (something else), "stick together." Of persons in embrace, c. 1600. Figuratively (to hopes, outmoded ideas, etc.), from 1580s. Of clothes from 1792. Related: Clung; clinging.
1670s, "act or state of sticking together," from French cohsion, from Latin cohaesionem (nominative cohaesio) "a sticking together," noun of action from past participle stem of cohaerere "to stick together," from assimilated form of com "together" (see co-) + haerere "to adhere, stick" (see hesitation).