sludge

American  

[sluhj] / slʌdʒ /

noun

1. mud, mire, or ooze; slush.

2. a deposit of ooze at the bottom of a body of water.

3. any of various more or less mudlike deposits or mixtures.

4. the sediment in a steam boiler or water tank.

5. broken ice, as on the sea.

6. a mixture of some finely powdered substance and water.

7. sediment deposited during the treatment of sewage.

8. Also called activated sludge.  Bacteriology. sewage sediment that contains a heavy growth of microorganisms, resulting from vigorous aeration.

9. a fine, mudlike powder produced by a mining drill.

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sludge British  

/ slʌdʒ /

noun

1. soft mud, snow, etc

2. any deposit or sediment

3. a surface layer of ice that has a slushy appearance

4. (in sewage disposal) the solid constituents of sewage that precipitate during treatment and are removed for subsequent purification

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Other Word Forms

• de-sludge verb (used with object)

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.

Example Sentences

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

"We can't show directly that that's happening, but we think it's a reasonable way that they could be winding up in the air. Sewage sludge fertilizers have been shown to release similar compounds."

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

For the purposes of “Crime 101,” that thoroughfare is reimagined as a quick escape route rather than the choked pipeline of sludge more familiar from reality.

From The Wall Street Journal • Feb. 12, 2026

She handed it back to me with sludge dripping down the sides.

From "Rump: The (Fairly) True Story of Rumpelstilskin" by Liesl Shurtliff

Related Words

• goo
• grease
• gunk
• muck
• sediment
• silt
• slime