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?
Ari: Then what the fuck do u want Cunt Muscle!?