Green’s Dictionary of Slang

1. a foolish, stupid person.

1797		M. Leeson Memoirs (1995) III 199: Not Broadhead but Flathead you surely should be, / As you’re really a Flat, in the highest degree.
1884		(con. c.1840) ‘Mark Twain’ Huckleberry Finn 33: I think they are a pack of flatheads for not keeping the palace themselves.
1900		Sydney Sportsman (Surry Hills, NSW) 28 Nov. 2/2: The fashionable flatheads do not crowd the dress circle so much.
1905		Sun. Times (Perth) 4 June 1/1: An ornamented Perth barmaid is off with her most recent mash [...] while she was landing the flathead her shop teeth slithered into her swanky [and] she tugged the molars back to her tater-trap .
1912		S. Ford Trying Out Torchy 23: if the jabs I sent in didn't get home, nothin’ [...] them other flatheads could think up would have any effect.
1914		Bulletin (Sydney) 16 July 36/3: To each and every flathead who has been ‘hooked’ a cunningly worded epistle is despatched, stating that the recipient has been selected on account of some special qualification or other [...]. And here the mugs fall in.
1920		F.S. Fitzgerald This Side of Paradise in Bodley Head Scott Fitzgerald III (1960) 266: They’d let any well-tutored flathead play football.
1929		J. Lait Broadway Melody 99: ‘If I’d won I’d have been in nothing.’ ‘Nothing but the girl, you flathead!’.
1937		T. Thursday ‘Good Luck is No Good’ in Federal Agent Nov. 🌐 He was a fathead, a roundhead, and a flathead to boot.
1944		J. Archibald ‘It Could Only Happen to Willie’ in Popular Detective Apr. 🌐 In a minute they will have your name on a door in this healing hacienda [...] you flathead.
1945		Baker Aus. Lang. 130: Fools of one kind and another [...] lolly, dilpot, dolly pot, gaylo, drip, flathead, possum, gammy, gazob, gimp and gup.
1947		T. Thursday ‘Billy the Kidder’ in Blue Ribbon Western Nov. 🌐 The Zizzen Boys, followed by a flock of extra-special flatheads, enter the tent.
1953		Kramer & Karr Teen-Age Gangs 177: That slimy flathead, he looks more like a cobra then ever.
1966		New Statesman 6 May 654/2: Gobbledygook is the defence of the American intellectual aware of the hostile mockery of the surrounding flatheads.

2. an inhabitant of the Illinois-Ohio lowlands.

1837		Ripley Transcript (MS) 30 Nov. 2/4: Take notice, all ye [...] Corn crackers, Yankees, Flat Heads [...] Land Pirates or what not, that if anyone has squatted on my improvements [...] I will row you up salt creek.
1837		Ripley Transcript (MS) 30 Nov. 2/4: Take notice, all ye [...] Corn crackers, Yankees, Flat Heads [...] Land Pirates or what not, that if anyone has squatted on my improvements [...] I will row you up salt creek.
1966		Dakin Dial. Vocab. Ohio River Valley 2.453: There is ample evidence that it [=hoosier] was originally a pejorative term comparable to the Flatheads of the Illinois-Ohio lowlands [DARE].

3. (US prison) a police officer.

1918

H. Simon ‘Prison Dict.’ in AS VIII:3 (1933) 26/2: FLATHEAD. Policeman.

4. (US) a Pole.

1945

H.B. Darrach Jr. ‘Sticktown Nocturne’ in Baltimore Sun (MD) 12 Aug. A-1/1: The bars are known by the company they keep. Latins go to Spik sticks, Poles to Square (or Flathead) joints, English to Chickie dives, and so on.

5. (US) a Lithuanian.

1989

(ref. to 1930s–40s) N.Y. Times 13 Dec. 31/1: The many Lithuanians in the neighborhood were called, for some unfathomable reason, ’flatheads.’.

6. a Jew.

1956

A.S.C. Ker Vocabulary West Texas 368: Jew (nicknames)... Flathead [1 inf] [DARE].

7. a German settler in Dakota or Wisconsin.

1967–8

in DARE.