Top definition
When you take your buddy's granny and do the slamming granny in the fanny on her.
John: "Tom I did the gran slam on your granny last night. I was pretty wasted. Sorry man."

Tom: " That is Fucked up. Now I have to do the San Francisco bird feeder on your mom."
by Papa Smurf blue baller June 11, 2017
Get the mug
Get a gran slam mug for your Aunt Larisa.