Thick, soft, wet mud or a viscous mixture of liquid and solid components.
Possibly from Middle English 'slicch' meaning mud, or related to 'slush'. The word emerged in the 17th century to describe muddy deposits, later extended to industrial waste and metaphorical uses.
Sludge is onomatopoetic - the word itself sounds thick and sluggish, mimicking the substance it describes. It's become a powerful metaphor in music (sludge metal) and politics (cutting through bureaucratic sludge).
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