late 14c., transitive and intransitive, "make avowal or admission of" (a fault, crime, sin, debt, etc.), from Old French confesser (transitive and intransitive), from Vulgar Latin *confessare, a frequentative form from Latin confess-, past participle stem of confiteri "to acknowledge," from assimilated form of com "together" (see con-) + fateri "to admit," akin to fari "speak," from PIE root *bha- (2) "to speak, tell, say."
Its original religious sense was in reference to one who avows his religion in spite of persecution or danger but does not suffer martyrdom (compare confessor). Old French confesser thus had also a figurative sense of "to harm, hurt, make suffer." Related: Confessed; confessing. An Old English word for it was andettan.