n. 1 thick greasy mud. 2 muddy or slushy sediment. 3 sewage. 4 Mech. an accumulation of dirty oil, esp. in the sump of an internal-combustion engine. 5 Geol. sea-ice newly formed in small pieces. 6 (usu. attrib.) a muddy colour (sludge green). sludgy adj.
[ cf. SLUSH ]