impermanent (adj.)

"enduring, unchanging, unchanged, lasting or intended to last indefinitely," early 15c., from Old French permanent, parmanent (14c.) or directly from Latin permanentem (nominative permanens) "remaining," present participle of permanere "endure, hold out, continue, stay to the end," from per "through" (from PIE root *per- (1) "forward," hence "through") + manere "stay" (from PIE root *men- (3) "to remain").

Related: Permanently. As a noun meaning "permanent wave," by 1909. Of clothing, permanent press, in reference to a process designed to produce lasting creases in fabric," is attested from 1964.

1796, from impermanent + -ence. Impermanency is from 1640s.

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.