penance n.

c. 1300, penaunce, "religious discipline or self-mortification as a token of repentance and as atonement for some sin; sorrow for sin shown by outward acts under authority and regulation of the Church," from Anglo-French penaunce, Old French peneance (12c.), from Latin pænitentia "repentance," noun of condition from pænitentum (nominative pænitens) "penitent," present participle of pænitere "cause or feel regret," probably originally "is not enough, is unsatisfactory," from pæne "nearly, almost, practically," which is of uncertain origin. Transferred sense of "repentance, contrition" is recorded from c. 1300. A popular Old French form, later ousted by the clerical pénitence, which preserves more of the Latin word.