Scum
/skˈʌm/
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Definition
(noun) A layer of impurities that accumulates at the surface of a liquid (especially molten metal or water).
Sentence Examples
You will never find a more wretched hive of scum and villainy.
Stop right there, you criminal scum!
CEFR Practice Quiz
A layer of green ____ had formed on the surface of the neglected garden pond.