ass in a sling, to have/get one's

Sl. to be dejected or hurt; to be pouting. (Potentially offensive. Use only with discretion.) She's got her ass in a sling because she got stood up. So you didn't get a perfect score. Why do you have your ass in a sling?

To be in deep trouble. The ass referred to is not the animal but the vulgar term for buttocks. The expression probably originated in the American South in the nineteenth century, and it is thought to refer to a kick in the buttocks so strong that the victim requires the kind of sling used to support an injured arm. The saying was common by about 1930.