late 14c., "wanting self-restraint," from Old French incontinent (14c.) or directly from Latin incontinentem (nominative incontinens) "immoderate, intemperate, not holding back," from in- "not, opposite of" (see in- (1)) + continens (see continent (adj.)). Originally chiefly of sexual appetites. General sense of "unable to retain" is from 1640s; medical sense of "unable to control bowels or bladder, unable to restrain natural discharges from the body" is attested by 1828.
He was incontynent, and with fleschely lustes he consumyd alle his tyme. ["Speculum Sacerdotale," 15th century]