Medusa pussy is the kind you get from an older woman who would otherwise be considered a milf except she's too gross. This kind of bitch is a real bitch. She's ugly inside and out. She's jealous, self-entitled, and extremely conceited. Despite her seeming immaturity, she's anything but in bed. She knows what she wants. She knows what you want. When you get drunk, the pearl necklace you inevitably give her is the nicest thing she owns.
It's a reference to Madame Medusa from The Rescuers.
Tim: Hey how did that Halloween party go?
Jim: I'm not proud of it, but I got some medusa pussy from Sue's aunt.
Tim: lol wtf