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.
a 16c. Latinizing revision of the spelling of Middle English fecond, fecound (early 15c.), from Old French fecond,fecont "fruitful" and directly from Latin fecundus "fruitful, fertile, productive; rich, abundant," from *fe-kwondo-, suffixed form (adjectival) of PIE root *dhe(i)- "to suck, suckle," with derivatives meaning also "produce, yield."
Also from the same Latin root come felare "to suck;" femina "woman" (literally "she who suckles"); felix "happy, auspicious, fruitful;" fetus "offspring, pregnancy;" fenum "hay" (probably literally "produce"); and probably filia/filius "daughter/son," assimilated from *felios, originally "a suckling."
<a href="https://www.etymonline.com/word/infecund">Etymology of infecund by etymonline</a>
Harper, D. (n.d.). Etymology of infecund. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/infecund