late 14c., confessioun, "action of confessing, acknowledgment of a fault or wrong," originally in religion, "the disclosing of sins or faults to a priest as one of the four parts of the sacrament of penance," from Old French confession (10c.), from Latin confessionem (nominative confessio) "confession, acknowledgement," noun of action from past-participle stem of confiteri "to acknowledge" (see confess).
An Old English word for it was andettung, also scriftspræc. Meaning "that which is confessed" is mid-15c. Meaning "a formula of the articles of a religious faith, a creed to be assented to" is from late 14c. In the common law, "admission or acknowledgment of guilt made in court or before a magistrate," 1570s.