This is when you grab your erect penis and move your arm in a type of oscillatory motion. The meat being the penis and beating being another word for masturbation.
Jerry: Damn i wanna be beating my meat right now!
*Jerry proceeds to unbuckle his belt and take off his pants*
*Jerry firmly grasps his penis and starts beating it*
Ask an unsuspecting victim if they have seen a 'beating cat's heart', followed by exposing one's testicles and pumping in and out of one's fist to mimic a beating heart.
Jeffrey: Hey William! Have you ever seen a Beating Cat's Heart?
William: I don't believe I have Jeffrey, please enlighten me!