To keep one's plans, intentions, or tactics secret from everyone else. Refers to holding one's playing cards close to one's vest 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 playing her cards close to her vest. Sorry for not being more straightforward about my plans, but I'm playing my cards close to my vest for the time being.