Finishing yourself off when a girl cannot make you cum. eg. when shagging or getting a bj she cannot finish you off so you take your cock in your own hand and do the job correctly into their face
In the film "Time Cop" Van Damme travels back in time to save his wife and when he travels back, he sees himself pumping his wife and decides to wait until they are finished. In actual fact, what he should have done is gone in and told his wife to stand aside; that he knew what he liked and finished himself off. Hence, time-copping.