Quiz: Remember the definition of mal de mer?
The Commodore Business Machines logo, which strongly resembles a poultry part. Rendered in ASCII as "C=".
With the arguable exception of the Amiga, Commodore's computers are notoriously crocky little bitty boxes (see also PETSCII). Thus, this usage may owe something to Philip K. Dick's novel "Do Androids Dream of Electric Sheep?" (the basis for the movie "Blade Runner"; the novel is now sold under that title), in which a "chickenhead" is a mutant with below-average intelligence.