keep/play (your) cards close to (your) chest

to not tell anyone what you plan to do.

I never know what Martin's next move will be. He plays his cards close to his chest.

• not know if you are coming or going
• not know someone from Adam
• not know the first thing about
• not know the first thing about something
• not know the half of it
• not know the meaning of the word
• not know what hit
• not know what hit you
• not know where to put
• not know where to put yourself
• not know where to turn
• not know whether are coming or going
• not know whether to laugh or cry
• not know whether you are coming or going
• not know which way to turn
• not let the grass grow under feet
• not let the grass grow under your feet
• not lift a finger
• not long for this world
• not look a gift horse in the mouth
• not lose sleep over
• not lose sleep over something
• not mince words
• not mince your words
• not miss a beat