Part of speech: Adverb
used in negative statement to describe a situation that has existed up to this point or up to the present time
Example: So far he hasn't called; the sun isn't up yet
Thus far, So far, Until now, Up to now, Hitherto
The word "hitherto" comes from Middle English hiderto.
hither – A related word in the same word family.
to – A related word in the same word family.
Middle English had more flexible word order than Modern English because it retained more inflections.