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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.
by Throbbing Monster February 09, 2009
 
1.
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.
by Cee Tee November 16, 2010