Green’s Dictionary of Slang

(US) particularly or only of interest to men; thus it’s a guy thing.

1992		Eble Campus Sl. Apr. 4: girl/guy thing – a subject understood only by members of one sex. ‘Oh, you wouldn’t want to hear about those cars – it’s a guy thing.’.
2000		Observer Mag. 21 May 3: Then it’s ‘guy stuff’ all the way.
2006		G. Malkani Londonstani (2007) 219: — Yuk, that’s disgusting, says the backless dress lady. — It’s just a bit of guy talk, babe.