The Headless Chicken is a demoralizing sex-act where two people are in the piledriving position, then the guy pulls out and starts running around holding the chick by the underside of her knees yelling "Headless CHICKEN!" The woman also adds her own screams to the mix because she is being pulled around by the legs with her head on the floor. In this sence, the legs of the upside-down woman resemble the wings of a chicken and the vag resembles where the chicken's head used to be.
"Fred and Daphney were definitly doing it last night. I saw Fred and a pair of legs run by the window so i'm guessing it was the headless chicken, that shit's awesome."