[gmc]

scum // n. & v.
n.
1 a layer of dirt, froth, or impurities etc. forming at the top of liquid, esp. in boiling or fermentation.
2 (foll. by of) the most worthless part of something.
3 colloq. a worthless person or group.
v. (scummed, scumming)
1 tr. remove scum from; skim.
2 tr. be or form a scum on.
3 intr. (of a liquid) develop scum.
scummy adj. (scummier, scummiest).
[Middle English via Middle Low German, Middle Dutch schum from Germanic]

scumbag // n. slang
a contemptible or disgusting person.

This must be one of those words whose meaning is changing.