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The leetspeak arised by the word elite which was shortened to leet and written 1337 in leetspeak. Leetspeak can be hard to read and was used as an secret code of certain groupings in former computer scenes. Today, the leetspeak is mostly used ironically.
Leetspeak arose to prevent the automatic read out of mails and messages by algorithms. Machines can not detect the word S3cr3t as Secret, but humans can (lunguistic redundancy). Because of this modifications, leetspeak has a sort of protection against spamfilter working with a blacklist. Passwords using leetspeak are much safer than protecting from bruteforce-attacks with word-lists.