Green’s Dictionary of Slang

(Aus.) an ad hoc nickname for any particularly stupid person.

1973		Tharunka (Sydney) 8 Nov. 28/3: An important-looking fucker at the gates holds us up to warn us that we’re speeding (what’s it to you, captain cretin?).
2003		Google Groups: uk.rec.cycling 5 Nov. 🌐 I use a headtorch, which can easily be pointed directly at Captain Cretin in his MDG.
2013		J. van der Ruit Spud: Exit, Pursued by a Bear [ebook] This means I’ll be sharing a field with Captain Cretin on Saturday against St David’s.