scum
/skʌm; skʌm/ n
1 [U] layer of froth on the surface of a boiling liquid; layer of dirt on a pond or other area of still water （煮沸的液体表面的）泡沫; （池塘或其他静止水面上的）浮垢, 浮渣.
2 [pl v] (fig derog 比喻, 贬) people considered to be bad or contemptible 一群坏人或卑贱的人: You scum! 你们这群坏蛋!
She treats smokers like the scum of the earth, ie as the worst people there are. 她把吸烟的人都当作败类.
I wouldn't have anything to do with those scum. 我才不跟那些无耻之徒来往呢.