'Banana Fisting' is where you get a hand of bananas and fist a girls pussy with them. This experience stretches a girls pussy really well (if you're into that kinda thing).
Person 1: Do you wanna know how bananas get the red tip on the end of them in the super-market?
Person 2: How?
Person 1: Well they begin the Banana Fisting Process on a woman on her Period.