If you’re a man, and you need a place to put your things, it might occur to you that a large, nicely made wooden box would be the way to go.
But wait! If people see you putting things into a nicely made wooden chest, they might assume that it’s a hope chest, and from that, of course, it follows that you are living in hope of soon marrying a man. (hence the name.)
Oh what shame and disgrace! All because you needed a convenient storage container for your various manly items.
Ah, finally! A box made just for men. At long last, I can put my things into a box without any risk to my masculinity and heterosexuality.
…, uh. Unless “ManBox” turns out to be gay slang for something, which seems plausible but I’m not going to Google it.