Insult. The counter to Dirtbag.
Guy one: You dirtbag!
Guy two: Yeah? well you're a mudsack.
Noun. Slang word for money, especially that obtained by criminal, illegal, or generally shady practices, e.g. selling drugs, prostitution, slave trading. Can also mean a pay check while working as an illegal immigrant. Used first by people in the black market and the Mafia.
Additionally, To 'give someone the mudsack' generally means to pay someone for doing something sneaky for you that you didn't want to do yourself.
Jones: "So, Tommy, did you bring me the mudsack?"
Tommy: "Oh man! Jeez, I was too busy running away from the cops to pick it up!"