Green’s Dictionary of Slang

a fool, also attrib.

1803		‘C. Caustic’ Petition Against Tractorising Trumpery 81: And couldst thou, pertinacious B-----, But maul these mutton heads, most sadly.
1811		Lex. Balatronicum.
1825		J. Neal Brother Jonathan I 99: Peace, muttonhead!
1834		Ely’s Hawk & Buzzard (NY) 15 Mar. 1/1: Our mutton head French Barber.
1837		Staffs. Advertiser 28 Oct. 3/7: Insley called him out of the office by the appellation of ‘Mutton Head’.
1848		‘Ned Buntline’ Mysteries and Miseries of N.Y. II 83: Get out of the way old mutton-head.
1862		‘Artemus Ward’ Artemus Ward, His Book 196: J. Davis, it is my decided opinion that the Sonny South is markin a egrejus mutton-hed of herself!
1875		Western Times 3 May 4/2: ‘No, you mutton head!’ responded the other.
1886		Aberdeen Jrnl 3 July 8/2: I’ve a notion to wear the butt end of this musket on your fool head. You — muttonhead.
1895		‘Banjo’ Paterson ‘Shearing at Castlereagh’ Man from Snowy River (1902) 137: You clumsy-fisted mutton-heads, you’d turn a fellow sick.
1902		S.E. White Blazed Trail 160: They sent some low-lived whelp down there to hang our drive, and by smoke it looks like they were going to succeed, thanks to you mutton-heads.
1912		A. Berkman Prison Memoirs of an Anarchist (1926) 192: The muttonhead sent for th’ district attorney the same day, an’ signed a third confesh.
1919		Marvel 1 Mar. 6: We must be a precious parcel of mutton-heads if we can’t.
1922		Exeter & Plymouth Gaz. 23 Nov. 2/6: I’m such a mutton-head, I fear — / I feel so sheepish when you’re near.
1927		(con. 1900s) S. Lewis Elmer Gantry 106: You tell the mutton-heads that the Bible contains absolutely everything necessary for salvation, don’t you?
1934		C. Stead Seven Poor Men of Sydney 89: It makes me sick to see what a muttonhead you are, Greg.
1940		O. Strange Sudden Takes the Trail 244: If you mutton-heads hadn’t butted in, she’d ’a’ bin at the Dumb-bell hours back.
1954		Oh Boy! No. 18 12: That fooled you mutton-head.
1955		B. Appel Tough Guy [ebook] Georgie agreed uneasily. ‘Yeh—’ ‘Don’t yeh me like a muttonhead, Georgie’.
1972		R.A. Wilson Playboy’s Book of Forbidden Words.
1988		R. Doyle Commitments 92: He wasn’t what they’d expected; some huge animal, a skinhead or a muttonhead, possibly both.
1997		C. McPherson The Weir 32: You’d spot that big muttonhead anywhere.