Scum
Definition 1 of 2

Literal meaning
Scum refers to actual physical dirt on liquids like water or soup.

You'll need to skim the scum off the top of the boiling broth.
Cleaning context
Scum is often mentioned when discussing cleaning fish tanks, ponds, or cooking pots.

After the fish tank was cleaned, there was no scum left.
Negative perception
When 'scum' refers to dirt, it suggests neglect or unclean conditions.

The bathtub is filthy with soap scum.
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cruddirtfilthgrimemuckpolluteslimesludgy
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