What does Boxing Day have to do with boxing?
c.1400, "cave, mine, pit" (late 13c. in place names), from a Scandinavian source, cf. Old Norse grod "pit," or from Middle Dutch groeve "furrow, ditch," both from Proto-Germanic *grobo (cf. Old Norse grof "brook, river bed," Old High German gruoba "ditch," Gothic groba "pit, cave," Old English græf "ditch"), related to grave (n.). Sense of "long, narrow channel or furrow" is 1650s. Meaning "spiral cut in a phonograph record" is from 1902. Figurative sense of "routine" is from 1842, often deprecatory at first, "a rut."
1680s, "make a groove," from groove (n.). Slang sense is from late 1930s. Related: Grooved; grooving.
A rut, groove, or narrow depression or channel in a surface.
[fr the sense that a musician is in a definite and exciting track, has hit a perfect stride, when playing well, esp a solo; perhaps influenced by the grooves of a phonograph record]