Any girl who refuses to be away from her boy. Typically, she puts a damper on otherwise good times by just being there. The same term could be reversed such that the boy is the pain in the ass, applied to gay couples, or (less accurately) lesbian couples.
"You remember how cool Rob used to be? Too bad he has that nutswinger of a girlfriend, now."
"Dude, your girls a damn nutswinger. Don't invite her."
"Dude, grow someballs and tell your nutswinger girl to stay home."