Green’s Dictionary of Slang

1. (US campus) dirt between the toes.

1969

Current Sl. IV:1.

2. (Aus. prison, also gronkster) a large, oafish fellow; a thug or heavy; hence, a fool or idiot, general term of abuse.

[1973		Canberra Times (ACT) 1 Dec. 8/3: [advert] Like Cats? Then why not share a house with Gronk, the biggest cat in Canberra?].
1979		Tharunka (Sydney) 8 Oct. 5/2: ‘Police’ said the voice as the back door left via the front door and I was slammed up against the wall and rather impolitely told ‘Don’t move you junkie cunt’ by an extremely ferocious gronk (commonly known as a narc).
1989		J. Lambert handwritten slang list GRONK - idiot, fool, dag - (perhaps from a small toy type fingurine - hairy, generally featureless).
1992		Canberra Times (ACT) 28 Oct. 8/7: The booklet was very helpful, said Mr Cahill [...] For him it had also been useful, especially in its listing of ‘jail words you need to know.’ [...] They included: Dog (someone who tells on other people), Gronk (a dag).
1996		KID: Kids’ Internet Dictionary 18 Oct. n.p.: gronk: idiot, stupid, mainly said after someone has done something stupid. ‘What a gronk!’ Comments: Not a new word, been around for a while, used by teenagers, not an abusive word. Informant: Age: 15; School: Trinity Grammar School; State: NSW.
1991		F. Mac Anna Last of the High Kings 121: He’s a gronk [...] Nelson Fitzgerald does not hang around with gronks.
2002		H. Everingham in Sydney Morn. Herald The Guide 4 Mar. 14/4: [T]he homely Miranda is getting hit on at the local gym by some gronk.
2010		3D World (Sydney) 27 July 40/1: Idiot Dickhead: ‘Omg! What happened?’ / Pretty Stupid: ‘Are you ok babe?’ / Big Gronk: ‘Nawwww that’s terrible! What happened? Let me know if you need anything.

3. an act of defecation.

1999

Roger’s Profanisaurus 3 in Viz 98 Oct. 16: gronkn. R.A.F To have a shit. ‘Bandits at three O’clock,’ ejaculated Algie. ‘Great!’ replied Biggles. ‘I’ve got half an hour to have a big smelly gronk!’ (from ‘Biggles Drops his Fudge’, by Capt. W. E. Johns).