There’s a book by Paul Dickson, “War Slang: American Fighting Words and Phrases Since the Civil War, Second Edition“” 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 vehicle pictured in this blog below is actually a 1941 Ford 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.
If you are interested in finding out more about WW2 era jeeps then you might want to check out my book on the subject: BANTAM, FORD AND WILLYS-1/4-TON RECONNAISSANCE CARS.