Green’s Dictionary of Slang

(orig. US) an absolute fool, an idiot, an incompetent.

1851		A.T. Jackson Diary of Forty-Niner (1906) 136: Pard shut the book with a slam and said I was a lunkhead .
1865		‘Artemus Ward’ Among the Mormons in Complete Works (1922) 300: Hurbertson, the utterly stupid boy—the lunkhead, who never had his lesson—he’s about the ablest lawyer a sister State can boast.
1872		Leavenworth Wkly Times (KS) 13 June 1/3: We cannot take lunk-heads, chowder-heads and cabbage-heads as figure heads this campaign.
1889		World (N.Y.) 7 Aug. 10/3: From the seats in right field Anson is continually called a ‘lunkhead.’ a ‘duffer’ and an old ‘soak,’ while Ewing is lauded to the skies.
1895		S. Crane Red Badge of Courage (1964) 105: B’jiminey, we’re generaled by a lot ’a lunkheads.
1901		Warren Sheaf (Marshall Co., MN) 31 Oct. 4/2: The Confessions of a Lunkhead [...] All was gumps an’ all was lunkheads, only they didn’t know.
1911		Herbert Quick Yellowstone Nights 164: I don’t take any high-an’-mighty stand-off from a lunkhead that’s stole my melons [DA].
1914		Day Book (Chicago) 1 June 17/3: What an awful lunkhead he is.
1934		Wodehouse Right Ho, Jeeves 39: In espousing the cause of a lunkhead capable of mucking things up as Gussie had done, I had taken on a contract almost too big.
1937		D. Fuchs Low Company 271: Let’s try the soda store [...] Maybe the lunkhead went there.
1940		E. O’Neill Long Day’s Journey into Night Act I: You’re a fine lunkhead!
1954		Oh Boy! No. 19 8: One of these lunkheads thought to follow!
1960		(con. 1940s) G. Morrill Dark Sea Running 138: A surprise move always paralizes these big lunkheads.
1971		J. Simon Sign of Fool 28: Hey, you lunkhead, get your lazy ass up!
1990		S. Morgan Homeboy 305: She was talking about that lunkhead she married.
1998		P. Doyle (con. late 1950s) Amaze Your Friends (2019) 185: It was the lunkhead who’d chased me away from Lee Gordon’s office.
2001		Guardian Guide 17-23 Mar. 98: A macho Italian lunkhead unwittingly becomes the roommate of a suave, sophisticated, urbane gay man.
2005		J. Stahl I, Fatty 101: All she really wanted was some lunkhead to pop the question.