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.
late 14c., "advantageous, fit, proper to a purpose," from Old French expedient "useful, beneficial" (14c.) or directly from Latin expedientem (nominative expediens) "beneficial," present participle of expedire "make fit or ready, prepare" (see expedite). The noun meaning "a device adopted in an exigency, that which serves to advance a desired result" is from 1650s. Related: Expediential; expedientially (both 19c.).
Expedient, contrivance, and device indicate artificial means of escape from difficulty or embarrassment; resource indicates natural means or something possessed; resort and shift may indicate either. [Century Dictionary]