Outworked is not common but I have heard it, especially in boxing matches where one fighter outworks their opponent.

Do you mean "What does it mean" and I have never heard that before. Are you sure it wasn't overworked?

I have heard this phrase used to mean 'Make sure you work harder than your competitors and others in your team, so that you win/get promoted in your job'. As Kieren said, it is quite unusual. Not really standard English.