late 14c., from Late Latin indeterminatus "undefined, unlimited," from in- "not, opposite of" (see in- (1)) + determinatus, past participle of determinare "to enclose, bound, set limits to" (see determine). Related: Indeterminately.
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., determinen, "to settle, decide upon; state definitely; fix the bounds of; limit in time or extent," also "come to a firm decision or definite intention" (to do something), from Old French determiner (12c.) and directly from Latin determinare "to enclose, bound, set limits to," from de "off" (see de-) + terminare "to mark the end or boundary," from terminus "end, limit" (see terminus).
Meaning "render judgment" is from early 15c. Sense of "give direction or tendency to" is from early 15c. Meaning "to find (as the solution of a problem)" is from 1640s. Related: Determined; determining; determiner.