There are many explanations of the origin of the word "jeep", which have proven difficult to verify. Probably the most popular notion holds that the vehicle bore the designation "GP" (for "Gov. Purposes" or "General Purpose"), which was phonetically slurred into the word jeep. However, R. Lee Ermey, on his television series Mail Call, disputes this, saying that the vehicle was designed for specific duties, was never referred to as "General Purpose", and that the name may have been derived from Ford's nomenclature referring to the vehicle as GP (G for government-use, and P to designate its 80-inch (2,000 mm) wheelbase). "GP" does appear in connection with the vehicle in the mode TM 9-803 manual, which describes the vehicle as a machine and the vehicle is designated a "GP" in TM 9-2800, Standard Motor Vehicles, September 1, 1949, but whether the average jeep-driving GI would have been familiar with either of these manuals is open to debate.
This account may confuse the jeep with the nickname of another series of vehicles with the GP designation. The Electro-Motive Division of General Motors, a maker of railroad locomotives, introduced its "General Purpose" line in 1949, using the GP tag. These locomotives are commonly referred to as Geeps, pronounced the same way as "Jeep."...
Source: http://en.wikipedia.org/wiki/Jeep
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