/ skʌm; NAmE / noun
1.
[ U , sing. ] a layer of bubbles or an unpleasant substance that forms on the surface of a liquid :
Skim off any scum.
stinking water covered by a thick green scum
2.
( informal ) [ pl. ] an insulting word for people that you strongly disapprove of :
Don't waste your sympathy on scum like that.
Drug dealers are the scum of the earth (= the worst people there are) .
► scummy / ˈskʌmi; NAmE / adjective :
scummy water
scummy people dropping litter
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WORD ORIGIN
Middle English : from Middle Low German , Middle Dutch schūm , of Germanic origin.