That person, normally a female, who defines everything that warrents your scorn. This term should only be said to the person it describes if you know you will never want anything from her (or him), including, but not limited to, sex, booze, sex with her sister or friend.
When you use the term 'scum sucking motherfucking piece of shit' in relation to another person, the subtle implication is you don't actually like or respect them.
"You may have saved the whales throughout the 80's Uncle Pete in your charitable life, however, there's no getting around it, in this life right now, you are a scum sucking motherfuckingpiece of shit and there's no getting around that."