What does Boxing Day have to do with boxing?
Old English sum "some, a, a certain one, something, a certain quantity; a certain number;" with numerals "out of" (e.g. sum feowra "one of four"); from Proto-Germanic *suma- (cf. Old Saxon, Old Frisian, Old High German sum, Old Norse sumr, Gothic sums), from PIE *smm-o-, suffixed form of root *sem- (1) "one," also "as one" (adv.), "together with" (see same). For substitution of -o- for -u-, see come.
The word has had greater currency in English than in the other Teutonic languages, in some of which it is now restricted to dialect use, or represented only by derivatives or compounds .... [OED]As a pronoun from c.1100; as an adverb from late 13c. Meaning "remarkable" is attested from 1808, American English colloquial. A possessive form is attested from 1560s, but always was rare. Many combination forms (somewhat, sometime, somewhere) were in Middle English but often written as two words till 17-19c. Somewhen is rare and since 19c. used almost exclusively in combination with the more common compounds; somewho "someone" is attested from late 14c. but did not endure. Scott (1816) has somegate "somewhere, in some way, somehow," and somekins "some kind of a" is recorded from c.1200. Get some "have sexual intercourse" is attested 1899 in a quote attributed to Abe Lincoln from c.1840.