Definitions by Dr. Walter W.D. Merriweather

fumunda 

1. The waxy coating, thin skin or membrane found in between the scrotum and anus: he checked the consistency and odor of his fumunda before exiting the bathroom.

The slang term is a shortening of the phrase "from under", more specifically "from under my balls".

fumunda by Dr. Walter W.D. Merriweather March 18, 2003