what does FJ stand for?
"J" is indeed for Jeep.
The original designation was the BJ.
"In 1950, the Cold War turned hot. The People's republic of North Korea - supported by it's communist allies from China and the Soviet Union - invaded the Republic of South Korea. The ensuing battle would be supported by the presence of some 400,000 American Military personnel.
To move it's armed forces around the rugged Korean Peninsula, the U.S. Army needed vehicles. The Willys jeep may have ruled the day through WWII, but the army didn't think war surplus general purpose vehicles would cut it in this new terrain. Mountains cover more than 70 percent of the Korean Peninsula, which has a cold and wet climate.
The Army sought updated vehicles, and it wanted them produced as close as possible to the peninsula. So it went to Toyota with blueprints to the venerable and combat-proven Willys Jeep and worked with the Japanese company on changes needed to meet the challenges in Korea. These changes included the need for a slightly longer wheelbase, a slightly more compliant suspension, and a more powerful engine than the four cylinder motor in the WWII jeep.
The engine chosen was the 85 HP Toyota Type B, an inline, 3.4 liter six cylinder fed by a single barrel carburetor.
Officially, the vehicle was designated the Toyota Jeep, though it was better known as the BJ because it combined the B engine and a jeeplike body and chassis
At first, the BJ was considered inferior to the "real" jeep, but in July of 1951, Toyota Test driver Ichiro Taira drove a BJ prototype to the sixth of the 10 hikers checkpoints on Japan's 12,388 foot Mount Fuji. That was higher than anyone thought likely-or even possible -in a four wheel motor vehicle. Then inspired by a historic horseback riding feat accomplised centuries earlier, Taira drove to Okazaki City and up steep temple stairs.
Taira's drive drew attention to the BJ."
"When Willys objected to Toyota using the "Jeep" name outside Japan, Toyota opted for "LandCruiser"-a twist on the name of the British-built Land Rover that had become the post war standard among civilian, if not quite civilized, four wheel drive vehicles."
"In 1955, the original LandCruiser, the BJ, was in it's final model year and a new version, the FJ, was making its debut. This new Landcruiser had a revised body with a face inspired by a traditional Japanese warrior's mask."
Toyota FJ Cruiser
Dragon covered it well
Ultimately, in Toyota land, the J is meaningless. The letter(s) preceding the J indicate which motor is installed. Examples
BJ-40 - "B" - 4 cyl diesel
FJ-40 - "F" - 6 cyl gas (later 2F)
FJ-60 - "2F" - 6cyl Gas
HJ-60 - "H" - 6 cyl diesel
FJ-62 - "3FE" - 6 Cyl FI
FJ-80 - "3FE"
FZJ-80 - FZJ motor
Toyota's FJ Cruiser model is technically the GSJ15