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P31 means you flip the bird on a buddy or a whole squad because you no longer tolerate their dumbassness. It’s usually sent as a letter, note, or a text that makes you laugh while they cry.
My homie got P31’d because he kept stealing my fries at lunch.
She P31’d her old squad after they started roasting her in the halls.
He P31’d the group because they kept calling him 'Sir' like he was royalty.