Origin and history of indistinct

late 14c., "not identical, not the same," also "clearly perceptible by sense," past-participle adjective from obsolete distincten (c. 1300) "to distinguish one thing from another; make distinct," from Old French distincter, from Latin distinctus, past participle of distinguere "to separate between, keep separate, mark off" (see distinguish). Meaning "plain and intelligible to the mind" is from c. 1600. Related: Distinctness.

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.