sludge /slʌdʒ/ BrE AmE noun [uncountable]
[ Date: 1600-1700 ; Origin: Probably from slush ]
1 . soft thick mud, especially at the bottom of a liquid
2 . the solid substance that is left when industrial waste or ↑ sewage (=the liquid waste from toilets) has been cleaned
3 . thick dirty oil in an engine
—sludgy adjective