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It means you got the goods, like a hot dog at a county fair. You grabbed the biscuit, and now you're the king of the hill. It’s like saying you got the last slice of pizza and it was still warm.
'Nab that biscuit,' he said, then walked away like he just won the lottery.
She texted him, 'Nab that biscuit, I’m coming over.'
At the bar, he said, 'Nab that biscuit, buddy, you're doing great.'