have a card up your sleeve

Meaning

1. to have an idea, a plan, etc. that will give you an advantage in a particular situation and that you keep secret until it is needed

From the same entry

• best card
• get your cards
• give their cards
• hold all the cards
• hold your cards close to your chest
• lay your cards on the table
• on the cards
• play the… card
• play your cards right
• tip your hand
• the cards are stacked against you
• the cards are stacked in your favour