What is the name of the lower part of a turkeys leg?

If you are referring to the red flap of skin under the turkey's beak then that is called a WATTLE.

The wattle is both a mate attraction and a cooling system. Blood pumps through the wattle close to the surface of the skin and ambient air help to displace body heat cooling the bird down. Turkey's do not sweat , they pant in hot weather and they allow cooler air to move over the wattles to help keep them from overheating.