What language technique is in the quote "Simply because we were licked a hundred years before we started is no reason for us not to try to win"? What does Atticus mean by "licked a hundred years before"?

This is an example of an allusion; an historical allusion, to be precise. Atticus uses a well-known colloquialism "licked," meaning defeated, to refer to the South's defeat in the Civil War. The context of the allusion is a discussion with Scout concerning the forthcoming trial of Tom Robinson. Atticus knows full well that Tom doesn't stand a chance of being acquitted, given the deep-seated racial prejudice that exists in Southern society. But that doesn't mean that he's not going to try and give it his very best shot. He has a moral and legal duty to provide Tom with the very best defense possible, and that's precisely what he intends to do. In using this allusion, Atticus is drawing an analogy between himself and the Confederate forces during the Civil War, in that both have been defeated before the battle's even begun, but still need to fight with everything they've got.