Throwing your shoulders back in an effort to hide your man-boobs during sex.
With the large pizza still winding its way through Jim's large intestine he was more than a little aware of his recent weight gain. When he pulled off his t-shirt he stayed upright in the bed, flensing madly while making weird grunting noises.
The act of Jiggling a woman's breasts without her consent. Especially if her shirt has been ripped off or soaked in water.
Yo There was some mad flensing going down at the Peurto Rican parade this year.