The portion of the scrotum near the ass-ball-connection, most commonly used for hand warmth.
Man, today it was so cold I had to use my under ball to warm up my fingers.
under ball: define #2
The area between the base of the scrotum and the anus. (informal) taint. Coined as a play on the compound word underarm.
That girl has the face of Mama Cass, the body of Mama Cass, and the hair of my underballs. I’ll pass.