From Scandinavian 'slusk' (slop), originally ship-galley grease before meaning melting snow.
Partially melted snow or ice, forming a watery, muddy mixture.
Probably from a Scandinavian source akin to Norwegian 'slusk' (slop) or Swedish dialect 'slusk' (wet, slushy ground). Originally a nautical term for grease or fat from cooking aboard ship, the snow sense developed in parallel. Key roots: slusk- (Scandinavian: "slop, wet ground").