Green’s Dictionary of Slang

a provincial, small-town person.

1934		Webster’s New International Dict. (2nd edn).
1945		Sat. Rev. Lit. (US) 6 Oct. 8/2: His books started some Americans laughing at others and made it possible for people to realize that somebody else was a Main Streeter [DA].
1947		Time 27 Jan. 4/3: It has raised the hope of this Mainstreeter from Podunk to its highest ebb since the era of Wendell Willkie’s ‘One World’ [DA].