What does Boxing Day have to do with boxing?
late 13c., "make double," from Old French dobler, from Latin duplare, from duplus (see double (adj.)). Meaning "to work as, in addition to one's regular job" is c.1920, circus slang, from performers who also played in the band. Related: Doubled; doubling. To double up bodily is from 1814.
A blow on the stomach "doubles up" the boxer, and occasions that gasping and crowing which sufficiently indicate the cause of the injury .... [Donald Walker, "Defensive Exercises," 1840]
mid-14c., "amount twice as great," also "duplicate copy," from double (adj.).