Green’s Dictionary of Slang

(orig. US) a fool, a simpleton; also their foolish brain.

[1919		N.Y. Tribune 19 Jan. n.p.: They clatter, girn, mow - peabrains rattle, rattle, rattle].
1941		A. Hoffmeister Animals are in Cages 59: You— you— pea-brain! Get out of here! That's Shakespeare!
1958		E. Wolf A-boating We Will Go 14: I said to myself, ‘Maybe this boating stuff is going to be O.K.’ Then peabrain here — " He grabbed Junior by the scruff of his neck. ‘Peabrain clobbered the works’.
1959		I. & P. Opie Lore and Lang. of Schoolchildren (1977) 201: He is [...] a pea-brain, or a putty-brain.
1968		K. Brasselle Cannibals 230: Why couldn’t those other peabrains realize that Winant was the indispensable man?
1984		D. Jenkins Life Its Ownself (1985) 164: I don’t know what pea-brain scheduled that cocksucker!
1989		H. Rawson Dict. of Invective (1991) 49: peahead. The reference is not so much to the vegetable as to the alleged size of the brain.
2000		C. Cook Robbers (2001) 296: Nothing left for those strutting peabrains in D.C. to do but dip their tallywhackers and run for reelection.
2005		J. Stahl I, Fatty 231: Vicious pea-brains who’d kill you for your comb.
2014		Smith & Zippilli ‘She Died with Grace’ in ThugLit Jan. [ebook] He was plugging holes wherever his pea-brain thought they were, real or imagined.