Whether TWAIN really stands for that is reaching urban myth status.

www.twain.org used to say that it doesn't stand for anything, which
sounded like an organisation wanting to forget an embarassing past.
Now they say that it is derived from the expression "and never the
twain shall meet", and that a contest was held retrospectively to
find an acronym. They didn't like any of the suggestions, but
Technology Without An Interesting Name was one of the entries
and it has haunted them ever since. They quote the Free Online
Dictionary of Computing for added emphasis.

http://www.twain.org/faq.htm