Butt sex. When a guy sticks his penis into a lady's ass hole. The penis usually ends up covered in poopies and generally quite dirty. this is where the expression comes from. Cuz usually if you go 4 wheeling in the backcountry, your car will end up covered in mud. Mud resembles poop and the car is a metaphore for your penis.
Female Infidel: OMG! Timmy was so horny last night but i was on my period. I let him go four-wheelin in the backcountry. It smelt like poops and now i cant control my sphincter so i wear a diaper.
Infidel: I went four-wheelin in the back country yesterday. Ass hole is the second vagina.