What does Boxing Day have to do with boxing?
c.1400, earlier scluse (mid-14c.), a shortening of Old French escluse "sluice, floodgate" (Modern French écluse), from Late Latin exclusa "barrier to shut out water" (in aqua exclusa "water shut out," i.e. separated from the river), from fem. singular of Latin exclusus, past participle of excludere "to shut out" (see exclude).
1590s, from sluice (n.). Related: Sluiced; sluicing.