What does Boxing Day have to do with boxing?
Old English mangere "merchant, trader, broker," from mangian "to traffic, trade," from Proto-Germanic *mangojan (cf. Old Saxon mangon, Old Norse mangri), from Latin mango (genitive mangonis) "dealer, trader, slave-dealer," from a noun derivative of Greek manganon "contrivance, means of enchantment," from PIE root *mang- "to embellish, dress, trim." Used in comb. form in English since at least 12c.; since 16c. chiefly with overtones of petty and disreputable.
1928, from monger (v.). Not considered to be from Old English mangian. Related: Mongered; mongering (1846).