Impromptu

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mid 17th century (as an adverb): from French, from Latin in promptu ‘in readiness’, from promptus (see prompt).

wiktionary

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Borrowed from French impromptu.

etymonline

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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.