It is another word for a mans forearm, often used in relation to the size. When one is beating a chicken on the head, one makes the same movement as if one is masturbating (known now as Chicken Beating). And when one masturbates a lot, one has a large forearm.
Dave: "Bloody 'ell mate. Can tell you've been single for quite a while. Check out the size of your chicken beater!"
Bill: "I know man. Too much time on my hands. I've been cracking one out at least 3 times a day just to pass the time."