What does Boxing Day have to do with boxing?
late 14c., from Old French revengier, variant of revenchier "take revenge, avenge" (13c., Modern French revancher), from re-, intensive prefix (see re-), + vengier "take revenge," from Latin vindicare "to lay claim to, avenge, punish" (see vindicate).
To avenge is "to get revenge" or "to take vengeance"; it suggests the administration of just punishment for a criminal or immoral act. Revenge seems to stress the idea of retaliation a bit more strongly and implies real hatred as its motivation. ["The Columbia Guide to Standard American English," 1993]
1540s, from Middle French revenge, back-formation from revengier (see revenge (v.)).