In response to the question “Where is she?”, I’ve heard someone say, “She’s back the hall.” (Cf. “She’s back there.”) I understand the meaning to be something like “She’s down the hall,” “She’s in the hall,” or “She’s in some room connected to the hall.”
It seems that back is functioning as a preposition here, as it doesn’t quite fit the adverb usage in its entry at M-W.
Is this a common usage in the sense of being documented in a reliable reference work or documented as an idiom or variant outside normal usage? If so, what part of speech is back?
Update: I have found an analogous phrase — “Back the Alley”, a decor and gift store in Gettysburg, Pennsylvania (Facebook page). The proprietor told me the origin of the name: “We started out in an unnamed alley and told everyone to go back the alley to find us.”
I have some Pennsylvania parentage, so that may be why this construction sounds perfectly natural to me.
No, that isn’t grammatical English. Similar correct sentences would be “She’s in the back of the hall” or “She’s back in the hall” (note that these have different meanings). As you point out, it looks like the speaker was trying to use back as a preposition, but it can only be used as an adverb or noun.