pasture chubby

This happens when you are wearing tight jeans while riding through a pasture (or off-road) in a truck. Next thing you know, you got a full on chubby because your jeans are rubbing on your penis. This is caused by the truck rocking back and forth due to the rough terrain.

"Damnit, these jeans are too tight, now I have a pasture chubby."

her: "Babe, why is your penis hard?"
you: "It's not hard, I got a pasture chubby!"