What does Boxing Day have to do with boxing?
1727, "muddy," from slop (n.1) + -y (2). Meaning "loose, ill-fitting, slovenly" is first recorded 1825, influenced by slop (n.2). Related: Sloppily; sloppiness. Sloppy Joe was originally "loose-fitting sweater worn by girls" (1942); as a name for a kind of spiced hamburger, it is attested from 1961.