noun which may take the genitive case particle 'no'
Used as a prefix-like modifier for men's items, especially in fashion and retail contexts. Often contrasted with レディース.
See also: レディース
This store has a large men's section.
I consulted a friend who knows a lot about men's fashion.
Slangy noun meaning 'men' or 'guys', often used in casual conversation among young people. Can sound rough or jocular.
Yesterday's drinking party was all guys.
Those guys are always so energetic.
I bought a men's shampoo.
The men's department in this store is on the third floor.
男 is the plain, everyday word for 'man/men', while メンズ (sense 2) is a slangier, more playful way to say the same thing.
Borrowed from English 'men's', with the possessive 's' reanalyzed as part of the word in Japanese. The slang noun sense developed from the same source.