The expression on someone's face while they are conducting a physically strenuous activity, or when they receive some unpleasant news.
Ex1 "Hey, new guy! Wipe off that fuck-me-face and get back to work."
Ex2 Dan's reaction to the loss of field pay was a series of Fuck-me-faces
Ex3 "That ruck march was so long, I was making fuck-me-faces every 20 meters."