Many explanations of the origin of the word jeep have proven difficult to verify. The most widely held theory is that the military designation GP (for Government Purposes or General Purpose) was slurred into the word Jeep in the same way that the contemporary HMMWV (for High-Mobility Multi-purpose Wheeled Vehicle) has become known as the Humvee. Joe Frazer, Willys-Overland President from 1939 to 1944, claimed to have coined the word jeep by slurring the initials G.P. There are no contemporaneous uses of "GP" before later attempts to create a "backronym."

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A more detailed view, popularized by R. Lee Ermey on his television series Mail Call, disputes this "slurred GP" origin, saying that the vehicle was designed for specific duties, and was never referred to as "General Purpose" and it is highly unlikely that the average jeep-driving GI would have been familiar with this designation. The Ford GPW abbreviation actually meant G for government use, P to designate its 80-inch (2,000 mm) wheelbase and W to indicate its Willys-Overland designed engine. Ermey suggests that soldiers at the time were so impressed with the new vehicles that they informally named it after Eugene the Jeep, a character in the Thimble Theatre comic strip and cartoons created by E. C. Segar, as early as mid-March 1936. Eugene the Jeep was Popeye's "jungle pet" and was "small, able to move between dimensions and could solve seemingly impossible problems."

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The word jeep, however, was used as early as 1914 by US Army mechanics assigned to test new vehicles. In 1937, tractors which were supplied by Minneapolis Moline to the US Army were called jeeps. A precursor of the Boeing B-17 Flying Fortress was also referred to as the jeep.

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Words of the Fighting Forces by Clinton A. Sanders, a dictionary of military slang, published in 1942, in the library at The Pentagon gives this definition:

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Jeep: A four-wheel drive vehicle of one-half- to one-and-one-half-ton capacity for reconnaissance or other army duty. A term applied to the bantam-cars, and occasionally to other motor vehicles (U.S.A.) in the Air Corps, the Link Trainer; in the armored forces, the ½-ton command vehicle. Also referred to as "any small plane, helicopter, or gadget.

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This definition is supported by the use of the term "jeep carrier" to refer to the Navy's small escort carriers.

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Early in 1941, Willys-Overland demonstrated the vehicle's off-road capability by having it drive up the steps of the United States Capitol, driven by Willys test driver Irving "Red" Haussman, who had recently heard soldiers at Fort Holabird calling it a "jeep." When asked by syndicated columnist Katharine Hillyer for the Washington Daily News (or by a bystander, according to another account) what it was called, Irving answered, "It's a jeep."

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Katharine Hillyer's article was published nationally on February 19, 1941, and included a picture of the vehicle with the caption:

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LAWMAKERS TAKE A RIDE- With Senator Meade, of New York, at the wheel, and Representative Thomas, of New Jersey, sitting beside him, one of the Army's new scout cars, known as "jeeps" or "quads", climbs up the Capitol steps in a demonstration yesterday. Soldiers in the rear seat for gunners were unperturbed.

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Although the term was also military slang for vehicles that were untried or untested, this exposure caused all other jeep references to fade, leaving the 4x4 with the name.

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The original trademark brand-name application was filed in February 1943 by Willys-Overland. It is also used as a generic term as the lowercase "jeep" for vehicles inspired by the Jeep that are suitable for use on rough terrain.