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You know the joke and you're stuck on the outside. Used when someone is too dumb to get the punchline and you're too lazy to explain it.
You: What's so funny? Me: I J and you're on the O.
At the party, I J and you're on the O. You looked confused. I laughed like I won the lottery.
I J and you're on the O. You asked why. I said, 'Because you're a brain-dead potato.'