impromptu (adv.)

1660s, from French impromptu (1650s), from Latin in promptu "in readiness," from assimilated form of in- "into, in, on, upon" (from PIE root *en "in") + promptu, ablative of promptus "ready, prepared; set forth, brought forward," from past participle of promere "to bring out," from pro "before, forward, for" (see pro-) + emere "obtain" (from PIE root *em- "to take, distribute"). From 1764 as an adjective; as a noun from 1680s.

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Definitions of impromptu from WordNet
impromptu (n.)
an extemporaneous speech or remark;
a witty impromptu must not sound premeditated
impromptu (n.)
a short musical passage that seems to have been made spontaneously without advance preparation;
impromptu (adv.)
without advance preparation;
Synonyms: ad lib / ad libitum / spontaneously
impromptu (adj.)
with little or no preparation or forethought;
an impromptu speech
Synonyms: ad-lib / extemporaneous / extemporary / extempore / offhand / offhanded / off-the-cuff / unrehearsed