word-forming element meaning "not, opposite of, without" (also im-, il-, ir- by assimilation of -n- with following consonant, a tendency which began in later Latin), from Latin in- "not," cognate with Greek an-, Old English un-, all from PIE root *ne- "not."
In Old French and Middle English often en-, but most of these forms have not survived in Modern English, and the few that do (enemy, for instance) no longer are felt as negative. The rule of thumb in English has been to use in- with obviously Latin elements, un- with native or nativized ones.
early 15c., "ready, prepared (to do something), quick to act as occasion demands," from Old French prompt (13c.) and directly from Latin promptus "brought forth," hence "visible, apparent, evident, at hand," past-participle adjective from promere "to take or bring out or forth" (see prompt (v.)). Meaning "given or performed without delay" is from 1520s. Related: Promptly.
<a href="https://www.etymonline.com/word/impromptitude">Etymology of impromptitude by etymonline</a>
Harper, D. (n.d.). Etymology of impromptitude. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/impromptitude