Green’s Dictionary of Slang

farmer n.2

1. (US) a derog. term for a peasant, an unsophisticated country person, whether an actual farmer or not.

[UK]Hotten Sl. Dict. 131: Farmer [...] In London it is used derisively of a countryman, and denotes a farm-labourer, clodpole. Both senses are different from the general acceptation.
[US]St Louis Post-Dispatch (MO) 3 Dec. 17/7: ‘A farmer’ [is] any man who does not know anything, coming of course from the cockney contempt of the country and countrymen .
[US]C.L. Cullen Tales of the Ex-Tanks 349: It was fierce, the way I got jerked around for a farmer that time.
Greenough & Kittredge Words 285: In this country [...] ‘farmer’ is sometimes jocosely applied to a greenhorn, or to a person who has made himself ridiculous, particularly by awkwardness or stupidity [DA].
[US]Ersine Und. and Prison Sl.
[US]W.R. Burnett Asphalt Jungle in Four Novels (1984) 211: Now you . . . farmer! Get your hands up!
[US]E. De Roo Young Wolves 134: Those farmers. They coulda used that time better tryin’ to score.
[US] in S. Harris Hellhole 141: And she’s such an old farmer, all she can think about is she don’t like my language.
[US]H. Rawson Dict. of Invective (1991) 192: As for country dwellers, in addition to hick, names and nicknames that have been used disaparagingly as generics include: [...] John Farmer (or John Family, John Hoosier, or simply John—or, for that matter, simply farmer).

2. a stupid or unsophisticated person.

[US]E. Townsend Chimmie Fadden Explains 41: Mr Paul is a farmer – I don’t tink. Why he’s slick as dey makes em.
[US]A.H. Lewis ‘Politics’ in Sandburrs 96: On d’ dead! I was farmer enough to t’ink I’d t’ank him for bein’ me guide.
[US]Ersine Und. and Prison Sl.
[US]N. Algren ‘Depend on Aunt Elly’ in Texas Stories (1995) 105: In his first fight there he gave away fourteen pounds against some farmer and cut him down in eight rounds.
[US]E. Gilbert Vice Trap 16: Carroll was nothing but a big farmer at heart.
[US]‘Hy Lit’ Hy Lit’s Unbelievable Dict. of Hip Words 14: farmer – A square.
[UK]Guardian G2 21 Aug. 4: I worked in a couple of those bars where you hustle champagne. They were business men, they weren’t naive farmers.

3. (US black) recently arrived Southern farm workers who persist in their country ways despite the pressing sophistication of the Northern cities.

[US]C. Brown Manchild in the Promised Land (1969) 178: They hadn’t been in New York long, and they didn’t know anything. Most of them were really dumb — farmers.
[US]A. Baraka Tales (1969) 14: Enty and Mazique are playing bridge with the farmers.
[US]R. Klein Jailhouse Jargon and Street Sl. [unpub. ms.].

SE in slang uses

In compounds

farmer’s alliance (n.)

(US) pumpkin pie.

[US]L.A. Times 9 Apr. 5: ‘Wake up,’ he cried, ‘one brown stone front, side of a funeral; two Irish lemons with all clothes on; plate of punk; an easy smear of axle grease and draw one in the dark, cap it all off with a farmer’s alliance.’.
farmer’s beef (n.)

(US) illegally shot deer, butchered and eaten by its hunter.

[US] in DARE.
farmer’s haircut (n.) [the farmer’s outdoor life gives the tan]

(US) a short haircut that leaves a white strip of skin showing between the bottom of the hair and the tanned portion of the neck.

R.V. Cassill Dr Cobb’s Game 359: A man with a farmer’s haircut and a plain, good-natured face.
MJLF 10.150: Farmer’s haircut. A closely trimmed cut which leaves a band of light skin contrasting with the tanned portions of neck and face, especially above the ears [DARE].
J. Sandford Sudden Prey .n.p.: A bearded man came around the corner, Pioneer seed-corn hat pulled low over his eyes. He walked like a farmer, heavy and loose, and had a farmer's haircut.
B. Watson Aliens in the Prime of their Lives 194: A soft, pale, fat girl with pretty blond hair and a thin, pimply boy with a farmer's haircut.
farmers’ time (n.)

(US) 30 minutes fast.

J.W. Yoder Rosanna 199: They were all in the carriage ready to go very soon after eight o’clock, farmers’ time, and that is always at least a half hour fast [DARE].
farmer’s wine (n.) (also farm liquor) [joc. euph.]

(US) illicitly distilled whisky.

[US]Randolph & Wilson Down in the Holler 243: farm liquor: n. Ordinary homespun whiskey, neither aged nor artificially coloured.
[US]DARE 360: farmer’s wine, field whiskey.

In phrases

eat the farmer after the gentleman

to eat beef after grouse, considered a breach of etiquette.

[UK]R.S. Surtees Jorrocks Jaunts (1874) 260: Before you attack the grouse [...] take the hedge [i.e. edge] off your appetites, or else there won't be enough, and, you know, it does not do to eat the farmer after the gentlemen.