remorseful (adj.)

"intense and painful self-condemnation and penitence due to consciousness of guilt; the pain of a guilty conscience," late 14c., from Old French remors (Modern French remords) and directly from Medieval Latin remorsum"a biting back or in return," noun use of neuter past participle of Latin remordere "to vex, torment disturb," literally "to bite back, bite again" (but seldom used in the literal sense), from re- "back, again" (see re-) + mordēre "to bite," which is perhaps from an extended form of PIE root *mer- "to rub away, harm."

The sense evolution was via the Medieval Latin phrase remorsus conscientiæ (Chaucer's remors of conscience, also translated into Middle English as ayenbite of inwit). Middle English also had a verb, remord "to strike with remorse, touch with compassion, prick one's conscience" (late 14c.), from Latin remordere. Richard Brome's "Merry Beggars" (1641) delighted that they had 

No bargains or accounts to make,

No land or lease to let or take:

Or if we had, should that remore us

When all the world's our own before us[?]