Green’s Dictionary of Slang

(N.Z./UK gay) someone who hides their homosexuality very deeply.

2010		W. Ings ‘Trolling the Beat to Working the Soob’ in Int’l Jrnl Lexicog. 23:1 73: Contemporary terms like street oyster (a used condom), money shot (ejaculation), Narnia (an extremely closeted client), and Whale Rider (a worker who specialises in very fat men).
2021		P. Lawson Beautiful Ruin [ebook] ‘He’s not out.’ I cock my head. ‘Are you?’ ‘Not publicly, [...] But that man's so deep in the closet it's a fucking wonder he hasn't emerged in Narnia’.
2022		J. Calder on Twitter 18 Aug. 🌐 Jonathan Calder @lordbonkers. [...] I’ve heard ‘He’s so deep in the closet he's in Narnia’.