It was invented in the war, so they could get around easily without using too many parts. JEEP Stands for Just Enough Essential Parts.
The Jeep was originally a U.S. Military vehicle first produced in 1941 by Willys. It was invented to provide a vehicle that was 4 wheel drive, small, lightweight, simple, easy to repair, that could be used by the military to move troops quickly. The name Jeep, according to the U.S. Army does not mean Just Enough Essential Parts as the above answer says. That is absurd and 100% wrong. The word Jeep just means a small transport vehicle. It is even used by the Navy to describe small escort carriers, and the Army Air Corp Link Trainer.
JEEP is an adaption of the letters GP, standing for General Purpose, vehicle.