Mainly a British term, with more than one definition. Usually refers to a jerk, an idiot, but also can refer to someone who jacks off frequently.
In the more mainstream use, it refers to an idiot, or a real annoying person, a jerk.
However it can simply refer to the penis itself.
Though in more popular use, it refers to those who are really into masturbation.
Implies a more than casual acquaintance with jerking off, to the point of it being a more regular occurrence.