What does Boxing Day have to do with boxing?
"frolic, merrymaking," 1590s, originally gambolde "a leap or spring" (c.1500), from Middle French gambade (15c.), from Late Latin gamba "horse's hock or leg," from Greek kampe "a bending" (on notion of "a joint"), from PIE *kamp- "to bend" (see campus).
1580s; earlier gambade (c.1500), from Middle French gambader, from gambade (see gambol (n.)). Related: Gamboled; gamboling; gambolling.