undercarriage

noun

1. The supporting structural framework of a land vehicle (such as a car, truck, van, wagon, etc).

   “Near-synonym: chassis”

   “The solution to coach riding defects, at least, seems to require much more co-operative practical experiment by all engineering departments to achieve better sympathy between the vehicle body, its undercarriage and the track on which it rides.”

2. The landing gear of an aircraft.
3. (euphemistic, humorous)The genitalia.