What does Boxing Day have to do with boxing?
mid-15c., earlier holin (mid-12c.), shortening of Old English holegn "holly," from Proto-Germanic *hulin- (cf. Old Saxon, Old High German hulis, Old Norse hulfr, Middle Dutch huls, Dutch, German hulst "holly"), cognate with Middle Irish cuilenn, Welsh celyn, Gaelic cuilionn "holly," probably all from PIE root *kel- "to prick" (cf. Old Church Slavonic kolja "to prick," Russian kolos "ear of corn"), in reference to its leaves. French houx "holly" is from Frankish *huls or some other Germanic source.