When you get hard and/or are being shaved by your farmer/master and get sexual feelings toward him/her. Usually happens at the point in your sheepy life at the age of adolescence. Cotton Boners are natural things and should not be ridiculed or made fun of.
"Do you see Jimmy's Cotton Boner? It's HUGE!"
"Son, Cotton Boners are natural, don't worry about it"
"Dude! Quit shaving me I think I'm getting a Cotton Boner!"
"I fuck sheep..."