c. 1300, "the reception, uncorrupted, of the Virgin Mary into Heaven" (also the Aug. 15 Church festival commemorating this, Feast of the Assumption), from Old French assumpcion, asumpsion (13c.) and directly from Latin assumptionem (nominative assumptio) "a taking up, receiving, acceptance, adoption," noun of action from past-participle stem of assumere "take up, take to oneself" (see assume).
Meaning "minor premise of a syllogism" is late 14c. Meaning "appropriation of a right or possession" is mid-15c. in English, from a Latin use (Cicero). Meaning "action of taking for oneself" is recorded from 1580s; that of "something taken for granted" is from 1620s.