What does Boxing Day have to do with boxing?
c.1400, frequentative of swelten "be faint (especially with heat)," late 14c., from Old English sweltan "to die," from Proto-Germanic *swel- (cf. Old Saxon sweltan "to die," Old Norse svelta "to put to death, starve," Gothic sviltan "to die"), originally "to burn slowly," hence "to be overcome with heat or fever;" also the source of Old English swelan "to burn," from PIE root *swel- (2) "to shine, beam" (see Selene). For specialization of words meaning "to die," cf. starve. Related: Sweltered; sweltering.