sludge(noun, verb) noun1. the precipitate produced by sewage treatmentDefinition categories: substance, precipitate2. any thick, viscous matterSimilar word(s): goo, goop, guck, gunk, muck, ooze, slimeDefinition categories: substance, matter verb1. (intransitive, informal) to slump or slouch.2. (intransitive) to slop or drip slowly.