Green’s Dictionary of Slang

mutton-headed adj.

also mutton-brained, muttonhead, mutton-minded
[SE mutton]

stupid, foolish.

[UK]Cleland Woman of Honor III 29: A poor mutton-headed flock, ready to follow any bell-weather.
[UK]Grose Classical Dict. of the Vulgar Tongue n.p.: Mutton-headed. Stupid.
[UK]Lex. Balatronicum.
[US]R. Waln Hermit in America on Visit to Phila. 2nd Ser. 27: A mutton headed Aminidab.
[US]S. Smith Major Downing (1834) 219: He’s a real mutton-headed boy.
[Ire]Sligo Champion 10 June 3/The good old mutton-headed, know-nothing squireens3: .
[UK]Morn. Post (London) 19 May 6/1: I, myself, know a big mutton-headed bull-dog physiognomied Squireen.
[UK]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’.
[Ire]Cork Examiner 1 Jan. 2/6: What is that mutton-headed fellow doing there?
[UK]York Herald 25 Aug. 6/3: You mutton-headed scullion!
[UK]Star (Guernsey) 26 Apr. 4/4: Imagine the generous fury of the wasp. ‘Oh, thou mutton-headed fly!’.
G. Bartram People of Clopton 49: He were sich a mutton-headed fool theer were no valley in ootwittin’ him .
[UK]Star (Canterbury) 14 Jan. 2: What’s worse, good old mutton-headed English believers.
[UK]C.J.C. Hyne Filibusters 262: The ancients [...] were by no means the mutton-headed fools we moderns generally chose to consider them.
[Aus]Bulletin (Sydney) 18 Aug. 44/2: Send a wire to that mutton-brained, blundering idiot of a chief clerk at Puketapu.
[US]G. Bronson-Howard Enemy to Society 278: So you’ve let the burglar escape, have you, you mutton-headed incompetent?
[UK]Marvel 26 June 6: Oh, you mutton-headed chump, Sammy.
[UK]Wodehouse Right Ho, Jeeves 200: She had caused all the trouble by her mutton-headed behaviour.
[US]N. Algren Man with the Golden Arm 274: Only some muttonheaded Pokey.
[UK]Wodehouse Jeeves in the Offing 157: That unfortunate muttonheaded girl.
‘P. Alding’ Field of Fire 33: A slightly mutton-headed D.I.
[Aus]D. Maitland Breaking Out 169: You are a bloody [...] mutton-minded, pin-headed, dooble-daft [...] fart-faced flip of a fucking galah!
[US]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).
[US]J. Stahl I, Fatty 157: Have some muttonhead actor give a speech.

In derivatives

muttonheadedness (n.)

stupidity.

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