infamous adj.

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., "celebrated in public report, renowned, well-known" also "notorious, infamous," from Anglo-French famous, Old French fameus (Modern French fameux), from Latin famosus "much talked of, renowned," often "infamous, notorious, of ill repute," from fama (from PIE root *bha- (2) "to speak, tell, say"). A native word for this was Old English namcuð, literally "name-known." Catch phrase famous last words in the humorous sense "remark likely to prove fatally wrong" is attested from 1921 (early lists of them include "Let's see if it's loaded ... We'll get across before the train comes ... Which one is the third rail? ... Light up, it can't explode").