imprompt (adj.)

mid-14c., prompten, "to incite to action, urge," from the adjective or from Latin promptus, past participle of promere "to bring forth," from pro "forward" (from PIE root *per- (1) "forward") + emere "to take" (from PIE root *em- "to take, distribute").

The meaning "coach (someone), assist (a learner or speaker) by suggesting something forgotten or imperfectly learned or known" is from early 15c.; specifically in the theatrical sense of "to assist a speaker with lines" by 1670s. Related: Prompted; prompting.

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.