Not a sexual term, similar to ass plow. Ass Tractor usually refers to the entity that is doing the plowing.
Man, that bitch took a ride on the ass tractor when her boss found out she was smoking weed on her lunch break.
An insult based on the combination of transportation methods and parts of the human anatomy, a great and innovative way to offend someone
1) Im going to heard your ass into a farm of pain with my ass tractor from hell BITCH