One who deliberately (or inadvertently sometimes) fucks their grandma. Note the difference between this and a "grand" mother fucker.
Joe: Steve, where were you last night?
Steve: Ahh .. umm.. I was out playing with the dog.
Joe: Don't lie, you incestuous grandmother fucker! I have it all on tape!
Steve: WTF?
Tim: i don't know but when I got back to her house her grandchildren were running all over the place so I took her to the basement, put a sock in her mouth and fucked her. After about 10min her grand kids came down and started watching.
Also known as your grandfather.
Named for mating with his wife to produce children who then become parents of his grandchildren.
Ben: Me and my motherfucker just went to see my grandmotherfucker for Motherfucker's Day (also known as Father's Day).
Gregory: My motherfucker (or my father) is such a charm and treats you with love.