What does Boxing Day have to do with boxing?
Old English werig "tired," related to worian "to wander, totter," from West Germanic *worigaz (cf. Old Saxon worig "weary," Old High German wuorag "intoxicated"), of unknown origin.
Old English wergian (intransitive), gewergian (transitive), from the source of weary (adj.). Related: Wearied; wearying.