The act of secret masturbation. Never done for voyeurism or sport. Usually done in the comfort of your dorm room on your friends desk when you think they are in class.
When you walk in on your roommate and he is crouched over, he is definitely scissor fisting!!
Code for masturbating when getting caught by friends. Usually a last resort.
Mark walked in on me beatin' it and I couldn't think of anything so I just yelled out "SCISSOR FIST!".