Is this a Spanish expression?
I came across a sentence that said "... solo calentar el asiento a trabajo," which means literally to "heat the seat alone at work". I'm wondering if maybe that's an idiom or something, like maybe to just come to work and do nothing?
"Calentar el asiento" is indeed an idiom, and it means to not do anything. As in that the only thing they do is warm the seat. For example, a common complaint could be that the boss doesn't do anything! He get's his salary for warming his seat! Or you could say it about a student, sure - he shows up in class but he doesn't pay any attention and doesn't do anything. He only warms the seat!