GEEP

At this point, the car manufacturer Jeep is a household name, but almost nobody in the 1930s had ever heard of one, and linguists have a hard time explaining its etymology. The most widely held theory is that it's a slurring of the initialism G.P., which was an assembly line code and was stamped on the sides of the open-air military vehicle that was widely used during World War II. Some thought this stood for General Purpose or Government Property and created the nickname along with their backronyms. This formation may have been partially or fully influenced by Eugene the Jeep, an animal in the Popeye comic strip that could only say the word "jeep". To confuse things even further, there are scattered mentions of troops in the US Army using the word to refer to unused equipment and uninitiated soldiers since World War I. Most likely, it's several of these and other possibilities mixed together, but we can't know for sure.