Green’s Dictionary of Slang

(US/Aus.) a stupid person.

[1912		Day Book (Chicago) 28 Dec. 10/2: He may have a dome of solid ivory and a brain of lard, but [etc.]].
1915		Sport (Adelaide) 21 May 3/7: It was at Uraidla social, / Where Lardhead took his stand. / My word he dribbled a bit, / But said he, ‘I look grand’.
1920		American Photo-engraver 12-13 372/1: Another viewpoint on the marriage question is cited when ‘Lardhead’ Marburger tried to smuggle a case of “medicine" past his wife into the house.
1936		J.T. Farrell World I Never Made 453: Fisher and Mostil are lard-heads. You can’t tell them a damn thing.
1943		H.A. Smith Life in a Putty Knife Factory (1948) 162: If you are a cultural lard head like myself.
1951		D. Stivens Jimmy Brockett 144: The trouble with the lardheads I have to work with, they never do anything exactly right.
1963		B. Hecht Gaily, Gaily 19: You lard head, I’m speaking about after you’re hanged.
1984		I. Doig Eng. Creek 211: Earl, you lardbrain.
1989		H. Rawson Dict. of Invective (1991) 232: lard. Fat, in various combinations, e.g., lard ass, lardball, lard bucket, and tub of lard, all referring to a grossly overweight person, and lardhead, to a dumb one.
2004		R. Cooley When Corruption Was King 109: A few other Mob lardheads were ‘elected’ alderman [...] but the real reins of power passed to Pat Marcy.