What does "I'd watch your back" mean? I watched a movie named Last Chance Harvey recently. Harvey is heading to London to join his daughter wedding. One of his colleagues tells him that their boss wants he to represent his own idea when Harvey is not here. And he said: "I am not doing myself any favor here, Harvey. I'd watch your back." What does "I'd watch your back" mean? Does it mean that he wants Harvey to be back soon? or something else?