Green’s Dictionary of Slang

(US) a farmer or countryman.

1877		in H. Asbury Gem of the Prairie (1940) 134: May Willard, the pocketbook snatcher [...] went through a granger on the West Side lately.
1882		G.W. Peck Peck’s Sunshine 229: When they palm off twelve grown persons onto a granger, in a sweat box like this, I had rather go to camp.
1889–90		Barrère & Leland Dict. of Sl., Jargon and Cant.
1909		W. Irwin Confessions of a Con Man 15: He thought I was a poor granger from Texas who had sold my farm.
1920		Ade Hand-made Fables 161: It does strike me that they ought to lay off us Grangers that are hard pushed.
1931		C. Falconer ‘The Pot-Rustler’ in Botkin Folk-Say 321: If a ‘granger’ lived within riding distance the wrangler was sent with the fore quarters to swap.
1936		R.F. Adams Cowboy Lingo 197: The farmer [...] was called a ‘granger’ in the Northwest.