Green’s Dictionary of Slang

meathead n.

[SE meat + -head sfx (1), implying that solid flesh, rather than brains, occupies one’s skull]

1. (US) a stupid person; thus as a term of address.

[US](con. 1917–18) C. MacArthur War Bugs 140: We bawled them out for a lot of meat-heads.
[US]‘Curt Cannon’ ‘The Death of Me’ in I Like ’Em Tough (1958) 101: You must take me for a meathead, Semmler.
[US](con. 1944) E.M. Nathanson Dirty Dozen (2002) 359: I’ve always been glad it was me and not some meathead you talked to.
[US](con. 1940s) E. Thompson Tattoo (1977) 173: Hey, that’s what I’ve been trying to tell a bunch of Missouri, Iowa and Oklahoma meatheads since I’ve been in.
[US]G. Tate ‘Adeva’ in Flyboy in the Buttermilk (1992) 113: She excused herself at one point to cold dismiss some meathead acting all loud.
[UK]K. Lette Llama Parlour 199: They’re in a kind of limbo-land. Not the macho meatheads they were brought up to be, or the Male Feminists they claim to be on telly chat shows.
[UK]K. Sampson Outlaws (ms.) 142: Al Stanton of the purple SLK. Meathead. Fucking bully.
[US]Mad mag. July 45: Lawsuits that pour in from bimbos and meatheads.
[Aus]L. Redhead Rubdown [ebook] Meatheads [...] Couldn’t handle that we were smarter than they were.
[UK]J.J. Connolly Viva La Madness 331: What was I thinking of? Getting that meathead Santos over here.

2. a general term of abuse.

[US]W.R. Burnett Asphalt Jungle in Four Novels (1984) 144: Look, meathead [...] no use us being sore at each other.
[US]L. Uris Battle Cry (1964) 117: O.K., you meatheads, take off your packs.
[US]USA Today 23 Dec. 04D: While Buffy hasn’t yet had a word achieve the breakout status of Seinfeld’s ‘yada, yada,’ or All in the Family’s ‘meathead,’ it is a rich source.
[US](con. 1954) ‘Jack Tunney’ Tomato Can Comeback [ebook] ‘My hand is pretty sore.’ ‘You mighta broke it on that meathead’s face’.

In derivatives

meatheaded (adj.)

stupid, foolish.

[US]St Paul Globe (MN) 20 Oct. 9/5: You big-eared, meat-headed chump.
[US]W.R. Burnett Asphalt Jungle in Four Novels (1984) 166: Whose would you take [...] some meat-headed tart’s? Or mine.
[US]‘Ed McBain’ Killer’s Wedge (1981) 26: Not even meatheaded Captain Frick.
[Ire]P. Howard The Joy (2015) [ebook] One of those meat-headed gym freaks, who spends hours lifting weights because he thinks it makes him more of a man.