"unappeasable," early 15c., from Old French implacable, from Latin implacabilis "unappeasable," from assimilated form of in- "not, opposite of" (see in- (1)) + placabilis "easily appeased" (see placate). Related: Implacably.
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.
"appease or pacify," 1670s, a back-formation from placation or else from Latin placatus "soothed, quiet, gentle, calm, peaceful," past participle of placare "to calm, appease, quiet, soothe, assuage," causative of placere "to please" (see please). Related: Placated; placating; placatingly.