The singer brings in the boar's head, "bedecked with bays and rosemary," to help celebrate Christmas. Chorus: Caput apri defero, Redens laudes domino."
The Latin chorus translates as "[The] head of [the] boar I bring, giving praises to God."
This is said to be the "earliest English carol to appear in print"; Ian Bradley's _Penguin Book of Carols_ reports it to have appeared in van Wynken's _Christmase Carolls Newly Emprynted at London_ (1521). Since I have not seen the latter book, though, and no one else mentions that publication, I haven't listed that as an earliest date.
Folklore also has a rather fantastic account of the origin of the song: An Oxford student named Copcot was on his way to mass when attacked by a boar. He allegedly killed it by stuffing a volume of Aristotle down his throat (an act, it seems to me, more likely to kill a lazy student than a boar), then took the head to the cooks. - RBW