What does Boxing Day have to do with boxing?
mid-14c., "moderate in desires or actions, temperate, restrained," especially "abstaining from strong drink," also "calm, quiet, not overcome by emotion," from Old French sobre "decent; sober" (12c.), from Latin sobrius "not drunk, temperate, moderate, sensible," from a variant of se- "without" (see se-) + ebrius "drunk," of unknown origin. Meaning "not drunk at the moment" is from late 14c.; also "appropriately solemn, serious, not giddy." Related: Soberly; soberness. Sobersides "sedate, serious-minded person" is recorded from 1705.
late 14c., "reduce to a quiet condition" (transitive), from sober (adj.). Meaning "render grave or serious" is from 1726. Intransitive sense of "become sober" (since late 19c. often with up) is from 1820. Related: Sobered; sobering.
son of a bitch (1918+)