Scum is the substance that grows in a warm moist place, much like mold. Its slimy and slippery. So a bag full of scum is a very nasty, distasteful item. It usually refers to a person who has acted in a very dishonest and disreputable way. Someone who leads another person on by misrepresenting himself/or lying about very important issues. Also sleazebag- same thing. And scumbucket, also the same.
