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You say this when you're so pumped you're like the third wheel of agreement, and you don't care if people get annoyed.
I third that, you stupid idiots. This pizza is the best thing since sliced bread and a bad breakup.
I third that, I'm gonna yell it from the roof of a taxi. This movie is the worst, but I love it.
I third that, and I'm not even related. This game is so good it's gonna ruin my life.