This article is from the alt.usage.english FAQ, by Mark Israel misrael@scripps.edu with numerous contributions by others.
"Typo" is related to, but does not come from, the verb "to type".
It is short for "typographical error", which, of course, could
refer to any error made by a typographer. (The humorous but useful
hackish coinage "thinko", used for when the person typing was
*thinking* of the wrong thing, pretends that "typo" does come from
"to type". The Jargon File also gives "mouso", a ubiquitous kind
of error in this point-and-click era.)
Arguments of the form "It couldn't have been a typo, because
those two keys are nowhere near each other on the keyboard" are a
bit tiresome, especially when one keeps the true etymology of "typo"
in mind.
 
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