It is used to describe the backside of somebody. It also Can mean a sexy Butt or nice curves on the back.
That Boy's Backcountry is very muscular.
Bobbi Billard Has a Fucking Hott Backcountry, I want to bang her.
After my freind Screwed a prostitute, he gave her a blumpkin and her Backcountry sweled like a balloon.