1. Undercarriage is what
planes land on. Called this, presumably, because it's carried under the plane.
Normally this will be their wheels, but sometimes it's skis (in the case of plains that land on snow) or floats (in the case of planes that land on water).
2. A euphemism for male genitalia, since it's also carried under the body. Assuming you're the right way up of course.