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).
Derivatives:
sludgy adj.
Etymology: cf. SLUSH