Internet slang. Two senses to be aware of: (1) neutral 'heterosexual man'; (2) the trendier weibo/小红书 sense, often shaded toward 直男癌 ('straight-male cancer') - i.e. tone-deaf, sexist, romantically obtuse. Tone of voice carries most of the weight.
culture
Pairs with 钢铁直男 ('steel straight guy', extremely oblivious) and 直男审美 ('straight-guy aesthetic', code for tacky design choices).
