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.