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There are several theories out about that. The most common is that it is just the way the GIs pronounced GP which stood for "General Purpose". Another theorie is that it evolved from the name GPW which is what Ford Called the vehicle. G is for Government, P is for its Wheelbase and W is for Willys-Overland.
I am just glad that Jeep has kept the famous grill as its signature for all of its vehicles.
Jeep
just empty every pocket
And I have, and use a Jeep.