[sludge] n [ME slugge, perh. alter. of slicche mud, slush; akin to OHG slih mire] (15c) 1: mud, mire; esp: a muddy deposit (as on a riverbed): ooze
2: a muddy or slushy mass, deposit, or sediment: as a: precipitated solid matter produced by water and sewage treatment processes b: muddy sediment in a steam boiler c: a precipitate or settling (as a mixture of impurities and acid) from a mineral oil
3: slush
5. -- sludgy adj