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1.
A Phrase used in calling someone's bluff or in casually suggesting that they are flat out lying to you or others. Can be used as a sarcastic command to deter a person from lying, or as a verb, generally in past or future tenses.
Command: "There's no way you survived a car crash like that. Story up, man, story up."

Verb: "I'll have to story up if she asks me where I've been."
by Backspin July 15, 2005
 
2.
A phrase used to imply that you can see through an untrue story or bash someone's untrue statement.
Kyle: Black people all look alike

Hanan: Story up!
by MrBlister101 December 15, 2008