mental fap
noun: The act of fapping to a mental image; usually one's pictured self engaged in fantastical/twisted sex acts with one or more people in his life. Many prefer the mental fap over the standard porn fap.
guy 1: "Dude.......last night I had a great mental fap about a black midget in a diaper and Joan Rivers...."
guy2: "Nice... you got that dude"
guy2: "Nice... you got that dude"