a film or layer of foul or extraneous matter that forms on the surface of a liquid.

refuse or offscourings.

a low, worthless, or evil person.

such persons collectively; riffraff; dregs.

scoria ( def. 1 ) .

to remove the scum from.

to remove as scum.

to form scum; become covered with scum.

noun

cleanliness

noun

algae , crust , dross , film , froth , residue , scruff , spume , waste , curs , dregs , lowest , mass , mob , proletariat , rabble , riffraff , rubbish * , scum of the earth , trash * , unwashed , vermin , dreg , lumpenproletariat , ragtag and bobtail , foam , froth.--v. despumate , recrement , refuse , slime , snake , trash