Green’s Dictionary of Slang

a fool, an idiot; often used affectionately as well as derog., also attrib.

1549		T. Chaloner (trans.) Erasmus Praise of Folie (1509) 67: I see many doltes, and fatteheddes woorshoppe suche stockes, in stede of the sainctes.
1599		Jonson Every Man Out of his Humour III ii: You sky-staring coxcombs you, you fat-brains, out upon you.
1840		R. Barham ‘Nursery Reminiscences’ in Ingoldsby Legends (1842) 135: You little Fat-head, / There’s a top because you’re good!
1854		Hertford Mercury 16 Sept. 3/3: When Rainbird passed, he, Lawrence, called out ‘Hollo Fathead,’ and Rainbird came back and asked who called him ‘Fathead’.
1861		(con. 1840s–50s) H. Mayhew London Labour and London Poor I 228/2: Why look at it, says I, fat-head – I knew I was safe – and see if there’s anything in it about the Queen or her coachman!
1876		Leamington Spa Courier 29 July 6/2: There appeared to have been some words between them, chiefly on account of the defendant’s son being called fat-head.
1880		Huddersfield Chron. 4 Dec. 2/6: Gobo [...] calls him ‘fathead,’ wears his hat, and does not display the slightest respect for his calling.
1885		Mrs J.H. Riddell Mitre Court II 124: He is a fathead—a great blundering John Bull.
1898		E. Pugh Tony Drum 26: I ain’t a-crying, fathead.
1900		Sporting Times 24 Feb. 5/5: Ma calls me many names; but her favourite is ‘lazy fathead’.
1904		Sydney Sportsman (Surry Hills, NSW) 29 June 2/3: She calls me ‘Fathead’ [...] a playful little touch.
1907		Sun. Times (Perth) 20 Jan. 4/8: Perhaps it is a trap to coax / Some fathead from his folly.
1908		Gem 17 Oct. 17: What are you thumping me on the back for, you fathead?
1912		Sport (Adelaide) 19 Oct. 13/2: They Say [...] That The living lamp post M.M. is still knocking around with fathead Bean.
1916		N. Douglas London Street Games 160: You’ve got to play something or other – unless you want to be a soppy fathead.
1918		Eve. World (NY) 19 Oct. 14/3: [cartoon caption] It’s a Cinch this Fathead isn’t Prejudiced against Himself.
1919		A. Baer Two & Three 15 Apr. [synd. col.] Cooking biscuits for fathead husband, who tells you to put your bonnets on backwards.
1925		Wodehouse Carry on, Jeeves 76: Bicky, though a stout fellow and absolutely unrivalled as an imitator of bull-terriers and cats, was in many ways one of the most pronounced fatheads.
1934		(con. 1920s) J.T. Farrell Young Manhood in Studs Lonigan (1936) 379: He knew he would make a fathead out of himself.
1937		R. Westerby Wide Boys Never Work (1938) 72: This three-ha’penny stamp fathead.
1945		(con. 1923) R. Westerby Mad in Pursuit 60: Don’t muck it up now, fathead.
1949		H. Miller Sexus (1969) 119: Now listen, you fathead.
1959		K. Waterhouse Billy Liar (1962) 144: ‘I’m pretending I’ve got flat feet,’ I said [...] ‘Fathead.’.
1967		F. Salas Tattoo the Wicked Cross (1981) 255: You know I ain’t no fathead.
1971		Wodehouse Much Obliged, Jeeves 22: I had always thought them rather fatheads.
1984		Columbia Journalism Rev. 5–6: [Talk show host Wally] George silences [guests] with shouts of ‘Shut up, fathead’ [R].
1997		N. Barlay Curvy Lovebox 151: Ain’t friskin’ him fathead he’s covered in shit.
2002		J. Deveraux Knight in Shining Armour 126: Instead, she had relinquished all control over the Stafford estates and married a fat-brain like Dickie Harewood.
2009		Observer TV 22 Feb. 10: They call each other ‘chap’ and sometimes even ‘fathead’.