What does Boxing Day have to do with boxing?
1560s, from Late Latin hippopotamus, from Greek hippopotamus "riverhorse" (earlier ho hippos ho potamios "the horse of the river"), from hippos "horse" (see equine) + potamos "river, rushing water" (see potamo-). Replaced Middle English ypotame (c.1300), which is from the same source but via Old French. Glossed in Old English as sæhengest.
Ypotamos comen flyngynge. ... Grete bestes and griselich ["Kyng Alisaunder," c.1300]