What does Boxing Day have to do with boxing?
early 13c., "untimely, unseasonable, unsuitable," from un- (1) "not" + tidy (adj.). Cf. West Frisian ontidich, Middle Dutch ontidich, Dutch ontijdig, Old High German unzitich, German unzeitig, Norwegian utidig "untimely, unseasonable, unfavorable." Meaning "poorly cared for, not neat" is recorded from mid-14c.