Green’s Dictionary of Slang

mutton-headed adj.

stupid, foolish.

1768		Cleland Woman of Honor III 29: A poor mutton-headed flock, ready to follow any bell-weather.
1785, 1788, 1796		Grose Classical Dict. of the Vulgar Tongue n.p.: Mutton-headed. Stupid.
1811		Lex. Balatronicum.
1821		R. Waln Hermit in America on Visit to Phila. 2nd Ser. 27: A mutton headed Aminidab.
1824		History of Gaming Houses & Gamesters 38: The late Earl was of the cart-horse breed, heavy, lump, mutton-headed, sixteen stone weight.
1833		S. Smith Major Downing (1834) 219: He’s a real mutton-headed boy.
1837		Sligo Champion 10 June 3/The good old mutton-headed, know-nothing squireens3: .
1840		Morn. Post (London) 19 May 6/1: I, myself, know a big mutton-headed bull-dog physiognomied Squireen.
1858		A. Mayhew Paved with Gold 291: The shepherd called the young tramps ‘a couple of blackguard thieves,’ and, in return, was termed ‘a mutton-headed fool’.
1859		Wkly Varieties (Boston, MA) 29 Oct. 3/2: Joe Carrol, a mutton-headed book-binder.
1868		Cork Examiner 1 Jan. 2/6: What is that mutton-headed fellow doing there?
1876		York Herald 25 Aug. 6/3: You mutton-headed scullion!
1887		Star (Guernsey) 26 Apr. 4/4: Imagine the generous fury of the wasp. ‘Oh, thou mutton-headed fly!’.
1897		G. Bartram People of Clopton 49: He were sich a mutton-headed fool theer were no valley in ootwittin’ him .
1899		Star (Canterbury) 14 Jan. 2: What’s worse, good old mutton-headed English believers.
1900		C.J.C. Hyne Filibusters 262: The ancients [...] were by no means the mutton-headed fools we moderns generally chose to consider them.
1904		Sydney Sportsman (Surry Hills, NSW) 26 Oct. 3/6: ‘I talk just as well as a blooming mutton-headed sheep like you!’.
1910		Bulletin (Sydney) 18 Aug. 44/2: Send a wire to that mutton-brained, blundering idiot of a chief clerk at Puketapu.
1911		G. Bronson-Howard Enemy to Society 278: So you’ve let the burglar escape, have you, you mutton-headed incompetent?
1920		Marvel 26 June 6: Oh, you mutton-headed chump, Sammy.
1934		Wodehouse Right Ho, Jeeves 200: She had caused all the trouble by her mutton-headed behaviour.
1949		N. Algren Man with the Golden Arm 274: Only some muttonheaded Pokey.
1960		Wodehouse Jeeves in the Offing 157: That unfortunate muttonheaded girl.
1973		‘P. Alding’ Field of Fire 33: A slightly mutton-headed D.I.
1979		D. Maitland Breaking Out 169: You are a bloody [...] mutton-minded, pin-headed, dooble-daft [...] fart-faced flip of a fucking galah!
1989		H. Rawson Dict. of Invective (1991) 49: muttonhead. Literally, a sheephead; from 1768 as mutton-headed (OED) and from 1804 as muttonhead (Dictionary of American English).
2005		J. Stahl I, Fatty 157: Have some muttonhead actor give a speech.

In derivatives

muttonheadedness (n.)

stupidity.

1998

G. Burn Happy Like Murderers 9: The mutton-headedness of Foresters resulting from inbreeding and incest [...] was until recent times a standing joke.