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."