A face which people make, often unaware, which gives the appearance that they are smelling a noxious odor. Often times, if a person is a fartface, this is the way a person's face looks even when they are not making a face. Also, if a person is a fartface, they also have an attitude or dillusions of being better than everyone around them, which is what causes their face to constantly contort this way.
Person 1: That bitchy looking girl looks like she smells something terrible!
Person 2: No, she's just a fartface.