What does Boxing Day have to do with boxing?
early 15c., "hairy, shaggy, bristling," from Latin horridus "bristly, prickly, rough, horrid, frightful," from horrere "to bristle with fear, shudder" (see horror). Meaning "horrible, causing horror" is from c.1600. Sense weakened 17c. to "unpleasant, offensive."
[W]hile both [horrible and horrid] are much used in the trivial sense of disagreeable, horrible is still quite common in the graver sense inspiring horror, which horrid tends to lose .... [Fowler]Related: Horridly.