To keep one's plans, intentions, or tactics secret from everyone else. Refers to holding one's playing cards close to one's chest in a card game, so as not to allow other players to see one's hand. We're all curious about what the boss has been discussing in those meetings with the lawyers, but she's keeping her cards close to her vest. Sorry for not being more straightforward about my plans, but I'm keeping my cards close to my vest for the time being.