Invisible Banjo 

statement describing act of masturbation, can be used for men or women

I was walking through the park and saw a peron playing the invisible banjo, I immediately called the police.

Invisible Banjo by meeshdoogle June 20, 2012

Playing the Invisible banjo 

Female masturbation, from the action of the hand as if strumming a banjo

Barbara's boyfriend couldn't make it that night, and with her being horny, she had to resort to playing the invisible banjo.

Playing the Invisible banjo by darfieldboy June 8, 2011