What does Boxing Day have to do with boxing?
late 14c., from Old French rigor "strength, hardness" (13c., Modern French rigueur), from Latin rigorem (nominative rigor) "numbness, stiffness, hardness, firmness; roughness, rudeness," from rigere "be stiff" (see rigid).
rigor rig·or (rĭg'ər)
Shivering or trembling, as caused by a chill.
A state of rigidity in living tissues or organs that prevents response to stimuli.