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.
1610s, "having the quality or power of determining," from Medieval Latin decisivus, from Latin decis-, past participle stem of decidere "to cut off; decide" (see decide). Meaning "marked by prompt determination" is from 1736. Compare decided. Related: Decisively; decisiveness.