The funniest thing about MGS3 is by this point the franchise was well-known for its sophisticated production values but Koijima still felt the need to retroactively develop the origins of names and titles he came up with in the 80s for an MSX game.
Big Boss was granted the title by Lyndon Johnson because he surpassed his mentor, The Boss, like America has fucking knighthoods but only for autistic spec ops soldiers
Metal Gears are called that because they serve to link the role of artillery and infantry like how a gear works to link the motion and movement of various moving parts of a machine
I guess it's better that it's cringe-inducing but sincere rather than somebody all-but-looking-at-the-camera and sardonically quipping that Naked Snake's the biggest boss around.