What is Grandmother Fucker?
1.
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?
See