Green’s Dictionary of Slang

(US) a rustic.

1899–1900		C.L. Cullen Tales of the Ex-Tanks 200: There’s been about forty or fifty of the junipers that shipped in Mare Island [...] jumped her down here.
1902		C.L. Cullen More Ex-Tank Tales 241: A juniper ’ud paste a silver dollar down on the bar for a drink.
1924		H.L. Wilson Professor How Could You! 313: I’ll bet you my share of the week’s takings we never set eyes on any one of those junipers again.
1944		R.F. Adams Western Words (1968) 167/2: juniper The western equivalent of hayseed.