it's not a typo, it's a shorthand for "/emote". at least, that's how I always understood it.
in reply to Re: proposal: implement /em to emote in the chatterbox
in thread proposal: implement /em to emote in the chatterbox
(emotion) A command used on talk systems and MUDs to indicate the performance of an action, usually a facial expression of emotional state.
The Free On-line Dictionary of Computing, Â© 1993-2007 Denis Howe