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.
mid-15c., fertil, "bearing or producing abundantly," from Old French fertil (15c.) and directly from Latin fertilis "bearing in abundance, fruitful, productive," from ferre "to bear" (from PIE root *bher- (1) "to carry," also "to bear children"). Fertile Crescent (1914) was coined by U.S. archaeologist James H. Breasted (1865-1935) of University of Chicago in "Outlines of European History," Part I.