Skum is 'foam', 'froth', same word as English scum, but when it's about unsavoury persons the Swedish word is avskum. The verb att skumma means both 'to foam' and 'to skim'. Skim seems to have taken a tour from its Germanic source via Old French into English.
Skummjölk is 'skimmed milk'
There is also an unrelated adjective, skum, meaning 'dark', 'dusky' or 'suspicious'. From that comes skymning - 'dusk' and the verb att skymma - 'to darken' or 'to block the sight' of something.