There’s a book by Paul Dickson, “War Slang” that says the jeep is “1, a small, low, khaki-colored car in general use in the Army. 2, a rookie; a recruit.OR you could go with his quote from another source, San Francisco Call-Bulletin, November 22, 1941 ” Do you know why those swift little army cars are called ‘jeeps’? It’s Model G-P produced by that automobile manufacturer–and G-P easily becomes ‘jeep’.” In 1941 Ford was producing a vehicle for the Army that was a model GP.

The unfortunate thing is that the War Slang book does not mention the jeep term being used during WWI or the inter-war period.  The book is available at Amazon for those that might be interested – War Slang: American Fighting Words and Phrases Since the Civil War, Second Edition.