1337 or leet is shortened form of 31337 or eleet. It is from a commonly known language today called 1337-speak or leet-speak. It originated from the Dead Cow Cult (31337) was the UDP port for Back Orifice. Now it is a language amongst teenage internet gamers, script-kiddies, and the like who really cant understand its out of date and to move on and try to crack a real encryption.
1337-speak: j00d i wtfpwn 4ll n00bz.
anyone with more than two brain cells: go get laid man.