When you absolutely plough the shit out of the side of someone's thigh with your knee, hence named because they will be walking like a grandad for the rest of the day.
"Hey Joffy, if you don't pass me a beer I'm gonna grandad the living shit out of you!"
by Frankoyo December 02, 2013
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An exclamation guaranteed to cause confusion and possible upset, particularly when used during coitus.
Man: "Oh yeah baby.....Mmm hmmm..."

Woman: "Oh yeah, oh yeah, oh yeah...."

Man: "Uh, uh, uh, uh...oh yeah....oh fuck...I'm gonna...I'm gonna...GRANDAD!!!!"

Woman: "WHAT THE FUCK IS WRONG WITH YOU?!?! Get off of me and get the fuck out of my house you sick freak!!"
by The Mongo Collective January 09, 2008
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where somone hits someone else in the area above the kneecap on the side of the left leg with their own knee. why? because its bloody funny (only if it works)!! the consequense of this will be the other person hopping on one leg shouting swear words,while u and ur m8s r laghing like hell. also some advice: always give a grandad when the other person is noot expecting and make sure u r standing up when doing it. there are adaptions of the grandad like a 'running grandad', the 'spinny' and a 'flying grandad'.
'oi m8 i think u'v got a grandad'!!!!!
by Mikey November 08, 2004
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When someone insults you then you tell your grandad
Your grandad that’s why
by Crjames September 27, 2019
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When you fuck him with a strap on and he yells his grandads name.
I dumped him cus he did the grandad.
by Jimby Bobbinson May 03, 2018
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