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Urinating in an open container while riding in a moving car. The phrase originated in 1835, when settlers riding the Oregon Trail were forced to relieve themselves in a large Stetson hat, to avoid stopping their wagons for fear of Indian attacks.
"Geez Jimmy, would you drive a little smoother? I'm filling up the stetson right now, but since you're driving like a maniac, it's splashing around everywhere."
by Ladderwalker January 27, 2009