1660s (adv.), 1764 (adj.), from French impromptu (1650s), from Latin in promptu "in readiness," from assimilated form of in- "into, in, on, upon" (see in-(2)) + promptu, ablative of promptus "readiness," from past participle of promere "to bring out," from pro- "before, forward, for" + emere "to obtain" (see exempt).