What does Boxing Day have to do with boxing?
"dribble from the mouth," early 14c., from Old Norse slafra "to slaver," probably imitative (cf. slobber (v.)). Related: Slavered; slavering. The noun is from early 14c.
"ship in the slave trade," 1830, agent noun from slave (v.). Meaning "person in the slave trade" is from 1842.