Gassed. Pooped. Running on empty. Exhausted.
Regardless of how you say it, it means you are having a tough day. It happens to everybody, myself included. Sometimes you can crush a Spike Energy drink, a Red Bull or even a cup of coffee, and feel good enough to get through you workout.
Then, there are those other days. You just don't have anything in you. What do you do? Do you go ahead and try and grind out the workout? Do you pack it up and head home?
If you decide to stick around, what happens? You try and grind it out; your warm up sets feel heavy, your feeler set feels like a max effort, and you get stapled by what was an easy training weight the week before. You get frustrated and wanna work through it, you try and push harder on the next set and tweak a muscle. Now you have had a shitty workout and hurt yourself. Fantastic!
Or, you could just call it quits and head home. Then, you would feel like a slug because you didn't get in a workout. You'd call yourself lazy and weak. You'd beat yourself up about it and feel like shit.
The other option, is to just do SOMETHING at the gym. Maybe you go through your first big lift (squats, deads, bench, push press, etc) at a much lower percentage than you are used to. Get your blood flowing and your muscles working, then see how you feel. If you feel like dog shit still, then beat it. Go home. Discretion is the better part of valor. If you feel ok, then try out your next exercise. Play it by ear.
Auto-regulation is a great thing when used appropriately. I'm all for toughing it out and working through things, but sometimes its really just not worth it to push through that shitty feeling.
Find something heavy. Pick it up.