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1.
One who fucks a grandmother
Steve Stifler in American Wedding is a grandmotherfucker, since he fucks Jim's grandmother.
by Greg Pugh August 14, 2003
 
2.
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?
by african_dude March 25, 2008
 
3.
1: The greatest mother fucker alive.

2: King/Queen of all mother fuckers.
You just got served you Grand Mother Fucker!
by Vulcanus January 17, 2005
 
4.
someone who, willfully or against his or her will, engages in sexual intercourse with someone who has grandchildren.

the person with grandchildren doesn't need to be the actual person's grandmother.
Rob: Dude, what happened at the bar last night?

Tim: didn't you see? I was talking to that old lady from the back there.

Rob: how old was she,

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.

Rob: WTF you nasty ass grandmother fucker!
by Rockman01 July 27, 2010
 
5.
A respected older motherfucker who has been promoted by excelling as a motherfucker or by fucking one or more grandmothers
John is now a Grand motherfucker.
by JAKINTEXES May 22, 2011