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1.
Slut muscles refers to any muscles that sluts use on a regular basis; inner thighs, hip flexors, as well as the pelvic (vaginal) muscles.
My slut muscles hurt....can you make them feel better?
by Livi-Whip November 11, 2010
 
2.
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.
by Fat Box March 08, 2012