What does Boxing Day have to do with boxing?
Old English gor "dirt, dung, filth, shit," a Germanic word (cf. Middle Dutch goor "filth, mud;" Old Norse gor "cud;" Old High German gor "animal dung"), of uncertain origin. Sense of "clotted blood" (especially shed in battle) developed by 1560s.
"triangular piece of ground," Old English gara, related to gar "spear" (see gar), on the notion of "triangularity." Hence also meanings "front of a skirt" (mid-13c.), and "triangular piece of cloth" (early 14c.).