From http://www.geocities.com/jeeptoys/history.htm:

The first coinage of the word "Jeep," as applied to a motor vehicle, occurred during WWI. According to Major E.P. Hogan, who wrote a history of the development of the Jeep for the Army's Ouartermaster Review in 1941, the word predated the controversy. "Jeep," he wrote, "is an old Army greasemonkey term that dates back to the last war (World War 1) and was used by shop mechanics in referring to any new motor vehicle received for a test." The word also found use in those days as a lessthancomplimentary term for new recruits. Jeep was still used in Army motor pools well into the 1930s when the next incarnation appeared and became the inspiration for many more Jeeps. On March 16, 1936, a comic strip character appeared that became an instant popular sensation. On that day, Eugene the Jeep was introduced to the already beloved Popeye comic strip by E.C. Segar. For those of us too young to have known Eugene well, he was described as being about the size of a dog, though he walked on his hind legs, and was a native of Africa. He subsisted on orchids and was said to be able to cross into the fourth dimension. He solved all sorts of complex problems for Popeye and Olive Oil, and always told the truth. The general public became so enamoured of Eugene the Jeep that his name quickly entered the slang vocabulary of the day. An average comment on an extremely capable person or thing might be, "Hey, he's a real Jeep!"

-Greg