Green’s Dictionary of Slang

(US) a fool.

1952		A. Godfrey 1 Jan. [radio show] Some clunkhead sent me three live quail [W&F].
1956		T. Thursday ‘The Big Squawk’ in Smashing Detective Mag. 15 Apr. 🌐 Look, clunkhead, you can get away with this butting-in via fiction on the radio, [...] but in real life, it doesn’t work.
1974		S.H. Taylor ‘Terms for Low Intelligence’ AS XLIX:3/4 205: [...] clunkhead.
1989		H. Rawson Dict. of Invective (1991) 47: clunkhead. Popularized by Arthur Godfrey on radio and television in the early 1950s (Dictionary of American Slang, 1975); possibly from clunker, for anything that is old, beat-up, and doesn’t work well.