There are several uses or meanings to this word, and in general many of the previous ones suffice. But only a few address its vintage slang connotation which is not currently in use much. To wit...decades ago, the word scum referred to semen. It was combined with the word bag to denote a condom - hence; scumbag. A scumbag was a necessary evil but not at all noble, ad was discarded immediately after use - having served its purpose and with no further use expected. In time, an undesirable person might be referred to as a "scumbag," and the emphasis on scum being a form of lowlife began to emerge. Because it was not considered profanity, the word eventually became acceptable for broadcast television, and over time, its original meaning evolved or morphed into what it is today. The use of the word has become commonplace, but persons of my generation - who still remember the original connotation, sometimes flinch a little when we hear children or parents saying it casually. It's not a bad word per se, but it's not a pretty one either. And God forbid that one would be considered to be a scumbag.
Hey Joey, do you have an extra scumbag with you? I ran out and I'm planning to fuck Mary tonight and she won't let me in without a rubber.