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."