A workout impostor is someone who walks around in workout or gym-like clothing to give the effect that they have worked out or gone to the gym today or are planning to work out or go to the gym today when in reality they have not or are not going to.
One usually does this to take a selfie and share it on social network or to make oneself feel better about their physique, while in reality they have done nothing to keep fit today.
Look at that girl in the nike shorts, under armour shirt and asics sneakers with a pony tail. She is totally a workout impostor. I didn't see her at the track or the gym at all today.