What does Boxing Day have to do with boxing?
c.1600, "victory involving one's own ruin," translating Greek Kadmeia nike, from Cadmus (Greek Kadmos), legendary founder of Thebes in Boeotia and bringer of the alphabet to Greece. Probably a reference to the story of Cadmus and the "Sown-Men," who fought each other till only a handful were left alive. Cf. Pyrrhic (adj.1).