Green’s Dictionary of Slang

1. a fool.

1929		Hostetter & Beesley It’s a Racket! 222: clunk—Stupid; slow-witted; foolish. Also, a fool.
1937		D. Fuchs Low Company 271: Just because he looks like a clunk, it don’t necessarily mean he ain’t got a head on him.
1947		T. Thursday ‘Sing Sing Sweeney’ in Crack Detective Jan. 🌐 Listen, you clunk.
1950		Sun (Sydney) 25 Jan. 2/6: That rugged fellow George Wallace has been touring Victoria and New South Wales [...] He discovered a new Australian character called a ‘clunk’ — the equivalent of the English village idiot.
1952		W.R. Burnett Little Men, Big World 20: The Bat’s pretty high on the boy, and he’s no clunk when it comes to fighters.
1963		F. Kohner Affairs of Gidget 29: If this makes me a clunk, so it makes me a clunk.

2. a man.

1948

R. Park Harp in the South in DSUE (1984).

3. (N.Z. prison) a form of prison-made cosh using a pool ball, D-sized batteries, or cakes of soap tied in a sock, a stocking, or a pillowcase.

2001

D. Looser Boobslang [U. Canterbury D.Phil. thesis] 44/2: clunk n. = dolly sense 3.