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."