Leet speak
Hackerspeak where E is 3.
Where it came from
Leet (from "elite") emerged in 1980s bulletin-board systems (BBS) as insider slang. Users swapped letters for look-alike digits and symbols: E→3, A→4, T→7, S→5. It signaled belonging to the "leet" crowd.
It later leaked into gaming tags, memes, and "pr0n"-style spelling. Pure subculture — a cipher only in the loosest sense.
How it works
There is no single rule; common swaps are A→4, E→3, I→1, O→0, S→5, T→7, and stylized variants like "0wn3d". Intensity ranges from mild to unreadable.
UWC applies a standard leet mapping to your text in the browser — flip it on for flavor, flip it off to recover the words.