Popliteal

popliteal \ˌpä-plə-ˈtē-əl or pä-ˈpli-tē-əl\ adjective: of or relating to the back part of the leg behind the knee joint

Etymology: New Latin popliteus, from Latin poplit-, poples knee joint, back of the knee

The medical term popliteal fossa refers to the knee pit, but English doesn’t have a common, non-medical term for that part of the body the way, say, German has with its Kniekehle.