This grey dirty 'Athletica' sweat pants was worn by Viggo Mortensen as The Man. It was worn throughout the movie, but this one seems to have been used on the "3st stage", during the "Militia gang" scene, as it is written inside. I think it is the scene when the cannibals are chasing them in the woods, where he's washing his son's head after killing one of the "militia gang" member. There indeed are some blood stains at the bottom of the pants. It comes from Premiere Props.
