Green’s Dictionary of Slang

prawnhead n.

(Aus.) a fool, a simpleton.

1961		W.R. Bennett Wingman 19: Answer me, prawnhead.
1966		Baker Aus. Lang. (2nd edn) 215: prawnhead, a simpleton or fool, a pejorative.
1987		K. Lette Girls’ Night Out (1995) 179: Prawnhead, a dentist-cum-oyster farmer, offered me something from his plate.
2002		ratemypicture.com 🌐 Hey prawnhead, suck my cock.

In derivatives

prawn-headed (adj.)

(Aus.) foolish, stupid.

1960		‘Nino Culotta’ Cop This Lot 94: You bloody useless prawn-headed mullet.
1963		‘Nino Culotta’ Gone Fishin’ 14: I have often been called a mullet. Sometimes I have been called a prawn-headed mullet.