Originates from the SED
and subsequently the Perl
construct for a regular expression
substitution: s/REGEXP/SUBST/ will replace whatever matches REGEXP with SUBST. Commonly used in chat contexts by people with a Unix background and/or wannabe hax0r
s to either correct their own previous typoes, or change the meaning of somebody else's statement.
Many people also tend to drop the final slash, either because they're lazy or because they don't really know where that expression comes from.