[This is a fast-access FAQ excerpt.]
"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"