sludge
Americannoun
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mud, mire, or ooze; slush.
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a deposit of ooze at the bottom of a body of water.
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any of various more or less mudlike deposits or mixtures.
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the sediment in a steam boiler or water tank.
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broken ice, as on the sea.
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a mixture of some finely powdered substance and water.
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sediment deposited during the treatment of sewage.
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Also called activated sludge. Bacteriology. sewage sediment that contains a heavy growth of microorganisms, resulting from vigorous aeration.
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a fine, mudlike powder produced by a mining drill.
noun
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soft mud, snow, etc
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any deposit or sediment
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a surface layer of ice that has a slushy appearance
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(in sewage disposal) the solid constituents of sewage that precipitate during treatment and are removed for subsequent purification
Other Word Forms
Etymology
Origin of sludge
First recorded in 1640–50; variant of dialectal slutch, slitch, Middle English slich “slime, wet mud” (compare its derivative slucched “muddy”); apparently of expressive origin
Explanation
Sludge is thick, damp, almost sticky stuff. If your friend dares you to climb through an old sewer pipe, you might want to check first to see if it's full of sludge. Sludge is often used to talk about the byproduct of a process like sewage treatment or oil refining, but you can use it for any viscous goop. Sludge floating in your fish tank is a sign that you should clean the water, and sludge inside a car's engine can affect the way the car runs. Sludge is a bit of a mystery — it may come from the Middle English slutch, "mud," or might be related to slush.
Vocabulary lists containing sludge
A Long Walk to Water
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The Devil's Arithmetic
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The Omnivore's Dilemma
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Example Sentences
Examples are provided to illustrate real-world usage of words in context. Any opinions expressed do not reflect the views of Dictionary.com.
The disaster unleashed toxic sludge into the Kafue River, contaminating land and leaving residents without clean water.
From The Wall Street Journal • May 1, 2026
If anything, after a few days spent dipping my T. rex hands into the cool primordial sludge of the dinosaur fandom, Panella’s sentiment feels like an understatement.
From Salon • Apr. 12, 2026
They frequently appear in wastewater and can end up in biosolid fertilizer, also called sewage sludge, which is produced during wastewater treatment.
From Science Daily • Apr. 11, 2026
The smog season regularly did a number on our mile-high mountains, wrapping them in a brown cloak of invisibility until wind and rain swept the sludge away.
From Los Angeles Times • Mar. 26, 2026
I imagined I could see the trolls in The Eastern Woods, slurping sludge and maybe eating apples.
From "Rump: The (Fairly) True Story of Rumpelstilskin" by Liesl Shurtliff
Definitions and idiom definitions from Dictionary.com Unabridged, based on the Random House Unabridged Dictionary, © Random House, Inc. 2023
Idioms from The American Heritage® Idioms Dictionary copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.
