remorse (n.)

late 14c., from Old French remors (Modern French remords), from Medieval Latin remorsum, noun use of neuter past participle of Latin remordere "to vex, disturb," literally "to bite back," from re- "back" (see re-) + mordere "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 (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."