1thick, soft, wet mud or a similar viscous mixture of liquid and solid components, especially the product of an industrial or refining process:the dumping of sewage sludge [count noun]:miscellaneous chemicals and treated sludges
dirty oil, especially in the sump of an internal-combustion engine.
2 [usually as modifier] an unattractive muddy shade of brown or green:a sludge green
3sea ice newly formed in small pieces.
adjective (sludgier, sludgiest)
early 17th century: of uncertain origin; compare with slush