Green’s Dictionary of Slang

1. (UK juv.) a fool.

1973		(con. mid-1960s) J. Patrick Glasgow Gang Observed 125: ‘Big Gonk’ (a word normally reserved for the inmates of the Lennox Castle Institution for the mentally handicapped).
2001		Guardian G2 3 Aug. 3: Huh! You gonk!

2. a contemptuous description of a prostitute’s client.

1977		D. Powis Signs of Crime 186: Gonk, a prostitute’s term for a client. Used contemptuously.
1978		W. Donaldson Balloons in Black Bag 156: How many customers? You know: johns, mugs, gonks, for Christ’s sake.
1989		J. Morton Lowspeak.
2024		J. Meades Empty Wigs (t/s) 886: [G]et money up front if suspecting gonk might be contemplate putting end it all suiciding himself off from high window.

3. an obsessive, an eccentric.

2000

Indep. Rev. 29 Feb. 7: They’re not huge corporations, they’re groups of teenage gonks in a back room in Kilburn!