The muscle inside the vagina (cunt).
This was used as an insult in the HBO hit series Entourage where character Ari Gold (played by Jeremy Piven) told character "E" that he was a cunt muscle for waking his family up in the early morning.
E: looks like i woke u up this time huh Ari?
Ari: Noo but u did wake my wife and kids dick head, Vinnie better be sitting in prison with a DUI or something is he?
E: No.
Ari: Then what the fuck do u want Cunt Muscle!?
Either a pantie thief or a person who thinks they are all knowing. Even when Proved Wrong. Also could be an unexpected/unannounced visitor Or children who frequently shit their pants.
I cannot believe that regurgitated Cunt Muscle Knocked on my door without warning, Stole my undies, and shat his pants! What a fucking queef!