What does Boxing Day have to do with boxing?
late 15c., from obsolete Middle English demean "handle, manage, conduct," later "behave in a certain way" (early 14c.), from Old French demener (11c.) "to guide, conduct; to live, dwell," from de- "completely" (see de-) + mener "to lead, direct," from Latin minare "to threaten," in Late Latin "to drive (a herd of animals);" see menace. Sense in English evolved from notion of "conduct, manage" (oneself). Spelling changed by influence of nouns in -or, -our.