Commonly-used slang for Eblanix, or the language (symbol set, really) used by elites (and in modern times, more and more magazine layout designers - grr!) using numbers and mathematical symbols to replace letters.

The conversion is thus: 1 = I or l, 3 = E, 4 = A, 5 = S, 7 = T, 8 = B, 9 = G, 0 = O, + = t

The term "hackspeak" in hackspeak would be displayed thus: h4ck5p34k.

I'm guilty of using l33t sp34k in the early 1980s. At least on the BBSes where I hung out, it came into use as a way to defeat profanity filters installed by the sysops. $#!T was a common construction.

It's funny seeing it these days -- unless you're using a monospaced font, most l33t sp34k doesn't look a whole lot like the "real" text it's supposed to be.