A. A good-natured nickname or banter between Heterosexual bros.
B. Someone characterized by being a slutty un-thinking sex fiend with low standards, similar to someone who thinks with their dick (basically any straight male).
A: What's up you dirty Slut Muscle.
B: My friend Ben is a major slut muscle...he wants to bang all the bitches, regardless of what they look like. He's a regular bro.