Someone who is undecided or wavering. The verb form is “to swither.” Comes from Scotland (topical!) and is in much more common use there. Almost onomatopoeic, or maybe just close to “slithering.”
More on this word from The Bottle Imp: http://bit.ly/1qYp9JZcan
Not necessarily on topic, but “The Bottle Imp” is a great name for a publication.