What does Boxing Day have to do with boxing?
Old English sunu "son, descendant," from Proto-Germanic *sunuz (cf. Old Saxon and Old Frisian sunu, Old Norse sonr, Danish søn, Swedish son, Middle Dutch sone, Dutch zoon, Old High German sunu, German Sohn, Gothic sunus "son").
The Germanic words are from PIE *su(e)-nu- "son" (cf. Sanskrit sunus, Greek huios, Avestan hunush, Armenian ustr, Lithuanian sunus, Old Church Slavonic synu, Russian and Polish syn "son"), a derived noun from root *seue- (1) "to give birth" (cf. Sanskrit sauti "gives birth," Old Irish suth "birth, offspring"). Son of _____ as the title of a sequel to a book or movie is recorded from 1929 ("Son of Tarzan").