What does Boxing Day have to do with boxing?
early 15c., variant of Middle English gnasten "to gnash the teeth" (c.1300), perhaps from Old Norse gnastan "a gnashing," of unknown origin, probably imitative. Cf. German knistern "to crackle." Related: Gnashed; gnashing.
Heb. harak, meaning "to grate the teeth", (Job 16:9; Ps. 112:10; Lam. 2:16), denotes rage or sorrow. (See also Acts 7:54; Mark 9:18.)