The term 'Hitching Aid' is a reference to the practice of hitchhiking, which is when a person stands by the side of the road and sticks out their THUMB in order to get a ride from a passing vehicle. The crossword solution for this term is 'THUMB, ' which is a five-letter word. This is because the THUMB is the most commonly used body part when hitchhiking, as it is the easiest way to signal to drivers that one is looking for a ride.