 apr 8urban dictionary word of the day    

go down on   

To go down on a woman is to start her off with nice, long, passionate kisses on the lips, and then down to her neck, and then down to her breasts where you have your pleasure with them, and then you kiss your way across her tummy, and finally, you have ultimate pleasure with orally pleasing her pussy.
 
"I want to go down on your sister."  

      not sure I would tell my buddy I wanted to eat his sisters pussy though...g