Lexicon: Fin To
First heard in Franklin, from JoeMeans about to, planning to, preparing to
Original (non-dialectical) form: fixing to Alternative form: finna to
"Fixing" as a verb meaning an intention dates back to the 14th century; common forms then would have been "fixing on" or "fixing for." The idea of active preparation, rather than mental resolve, developed in 18th century America.
This is common verbiage all over the south; I'm attributing it to the place I learned it.