Gonad, male: The male gonad, the testicle (or testis), located behind the penis in a pouch of skin (the scrotum). The testicles produce and store sperm and are also the body's main source of male hormones (testosterone). These hormones control the development of the reproductive organs and other male characteristics, such as body and facial hair, low voice, and wide shoulders.
The word "testis" is a Latin word that means "witness." In ancient times, a person providing testimony might validate it by holding the scrotum, his or another man's. "And the servant put his hand under the thigh of Abraham his master, and swore to him concerning the matter." (Genesis 24:9)