its not a language, its jargon, I think it originated from Hackers trying to not get their messages caught on search engines, I could be mistaken though. Thats how I heard it
but 1337 is where you replaces letters for different characters 1 = l, 3 = E, 4 = A, 5 = S and so on
Its called 1337 speak, used to be called 31337 speek, but 1337 sounded cooler, and ofcourse it means elite speak