What does Boxing Day have to do with boxing?
early 15c., from Latin rigidus "hard, stiff, rough, severe," from rigere "be stiff," from PIE *reig- "stretch (tight), bind tightly, make fast" (cf. Old Irish riag "torture," Middle High German ric "band, string"). Related: Rigidly.
rigidity ri·gid·i·ty (rĭ-jĭd'ĭ-tē)
The quality or state of stiffness or inflexibility. Also called rigor.
An aspect of the personality characterized by resistance to change.