What does Boxing Day have to do with boxing?
"to bite gently," c.1500, perhaps from Low German nibbeln "to nibble, gnaw," related to Middle Low German nibbelen, Middle Dutch knibbelen "to gnaw," source of Dutch knibbelen "to cavail, squabble." Related: Nibbled; nibbling.
1650s, "act of nibbling," from nibble (v.). As "a small bite," from 1838.