Ace up your sleeve

e.g. I don't know how Henry is going to get his mom buy him a bike, but I'm sure he has an ace up his sleeve.

Meaning: a surprise or secret advantage, especially something tricky that is kept hidden until needed.

Origin: Back in the 1500s most people didn't have pockets in their clothes, so they kept things hidden in their sleeves. Later on, magicians hid objects, even small live animals, up their sleeves and then pulled them out unexpectedly to surprise their audiences. In the 1800s dishonest card players secretly slipped a winning card, often an ace, up their sleeves and pulled it out to win the game when nobody was looking.

指意料之外的秘密手段，这个谚语源自在袖子中藏东西的魔术师和打牌出老千的人。