I've heard this one before from my conservative grandma, It's when a girl is doing something manly that the guy ""should"" be doing. Like if a girl is carrying in all the groceries while a guy is just watching someone would say "let [guy] do it, he's supposed to feel like a man"

This came up a lot as my sister is very much a 'do it yourself' kinda gal whereas her (now ex) boyfriend wasn't much of an initiative taker.

It's not about a guy not doing manly things, it's about stopping women from doing manly things.
(also note I'm using 'manly' in the stereotypical terms, not how I personally see them)