indiscretion (n.)

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.

c. 1300, dyscrecyounne, "ability to perceive and understand;" mid-14c., "moral discernment, ability to distinguish right from wrong;" c. 1400, "prudence, sagacity regarding one's conduct," from Old French discrecion and directly from Medieval Latin discretionem (nominative discretio) "discernment, power to make distinctions," in classical Latin "separation, distinction," noun of state from past-participle stem of discernere "to separate, distinguish" (see discern).

Phrase at (one's) discretion attested from 1570s (earlier in (one's) discretion, late 14c.), from sense of "power to decide or judge, power of acting according to one's own judgment" (late 14c.). The age of discretion (late 14c.) in English law was 14.

1788, from indiscretion + -ary.