What does Boxing Day have to do with boxing?
early 13c., "body part or other object from a holy person," from Old French relique (11c., plural reliques), from Late Latin reliquiæ (plural) "remains of a martyr," in classical Latin "remains, remnants," noun use of fem. plural of reliquus "remaining, that which remains," related to relinquere (perfective reliqui) "to leave behind" (see relinquish). Sense of "remains, ruins" is from early 14c. Old English used reliquias, directly from Latin.
in religion, strictly, the mortal remains of a saint; in the broad sense, the term also includes any object that has been in contact with the saint. Among the major religions, Christianity, almost exclusively in Roman Catholicism, and Buddhism have emphasized the veneration of relics.