A night in which friends gather together and get so crunk that after the partying, all they can physically do is sit on the couch like a grandma and rock back and forth.
Mike, Jordan, Zach, Robert, and Matt had a grandma night last night with a bottle of Vladimir. Zach passed out first. He was such a grandma.
by prohighfiver June 25, 2010
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It litterally means "I had sex with your grandmother last night."
It just removes the word 'sex' to make it shorter, like any other slang term.
Ryan: "Yo man, what were you doin' last night?"

Jack: "I had yer grandma last night"

Ryan: "What the hell man?! That's sick!"
by jeNANANANANAA BATMAN! April 11, 2009
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