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."