Green’s Dictionary of Slang

(US) a fool; thus adj. pestle-headed, foolish.

1891		Progressive Farmer (Winston, NC) 20 Oct. 6/3: Do not the pestleheaded Arnolds and Annaniases know that the know that the combined forces of Wall Street [...] will not deter us?
1915		Western Carolina Democrat (Hendersonville, NC) 7 Oct. 7/4: He sends a pestle-headed son off to a one-horse college. [He] gets a piece of sheep-skin [...] and with this [...] grafts a living.
1921		Indep. (Elizabeth City, NY) 15 July 4/1: The most pestle-headed, narrow visioned reader of this newspaper [etc].