have nothing left in the tank

To have no more energy, willpower, or motivation to continue. A: "Come on, we've just got another mile to go!" B: "No way, dude, I'm done. I've got nothing left in the tank. Their defense has put up an incredible fight all game long, but it's pretty clear by now that they just have nothing left in the tank. If we keep having to work 80-hour weeks to get this project done, we'll have nothing left in the tank by the time we reach the deadline.