Green’s Dictionary of Slang

mullet n.¹

(Aus./US) a fool.

1886		Melbourne Punch 10 June 9/1: But allowing the usual percentage for deadheads there ought to be enough mullets among that number to pay the remaining 25 sovs.
1900		Star (N.Z.) 19 May 2/3: The mater was a mullet if ever there was one. I used to tell her I was a real bad egg, but her vanity was so stupendous it would not ler believe me.
1922		Smith’s Wkly (Sydney) 2 Dec. 18/3: ‘I’ve been robbed,’ he roared. Up rushed another mullet with the same story.
1955		J. Blake letter 10 Oct. in Joint (1972) 70: If I don’t have bread, I may look round for a likely prospect (locally, ‘mullet’) to buy it for me. However, this entails the real danger that the mullet will want to come along with me, thus beginning the day with a spate of paralyzing conversation.
1963		‘Nino Culotta’ Gone Fishin’ 14: I have often been called a mullet. Sometimes I have been called a prawn-headed mullet.
1965		D. Pearce Cool Hand Luke (1967) 63: Ah got me a pair of mullets what think they can beat me out on the time.
1976		Eble Campus Sl. Nov. 4: mullet – a person who is gullible or easily persuaded.
1986		R.G. Barrett Real Thing 82: You two have got to be the biggest pair of Botany Bay mullets I’ve ever seen. You come in with your big gobs open every time.

In compounds

In phrases

like a stunned mullet (adj.) (also like a baffled mullet)

(Aus./N.Z.) dull, stupefied.

1940		Sporting Globe (Melbourne) 9 Oct. 2/6: All the week end I have been walking round like a stunned mullet wondering why I didn’t [...] have a headlong tilt at Ajax in the Melbourne Stakes.
1953		Baker Aus. Speaks 267: [Similes] Dullness: (looking) like a stunned mullet.
1972		Aus. Women’s Wkly 17 May 5/3: I sit like a stunned mullet from go to whoa.
1986		R. Beckett Dinkum Aussie Dict. 49: Stunned mullet: If someone has an expresssion like a stunned mullet that person is deemed to be both stupid and ugly.
1996		(con. 1964-65) B. Thorpe Sex and Thugs and Rock ’n’ Roll 19: I just lay there with my open mouth like a stunned mullet.
1999		Guardian Rev. 31 July 12: Guests [...] sit like baffled mullets wondering if they might get a word in.
1999		Guardian Guide 31 July–6 Aug. 23: They might look like a couple of stunned mullets.
2000		Hartford Courant (CT) sect. D 5 Sept. 27/4: G’Day from Down Under [...] Stunned mullet — Someone who seems to be out of it.
2003		McGill Reed Dict. of N.Z. Sl.
2004		L. Redhead Peepshow [ebook] I waited by the door like a stunned mullet.