have an ace/a trick up your sleeve

have an ace up your sleeve

have an effective resource or piece of information kept hidden until it is necessary to use it; have a secret advantage.
The ace is the highest playing card in its suit in many card games, so a cheating player might well hide one to use against an unwary opponent. A North American variant is an ace in the hole . The next two idioms are also based on this meaning of ace .
See also: ace, have, sleeve, up

have an ˌace/a ˌtrick up your ˈsleeve

(British English) (American English have an ˌace in the ˈhole) (informal) have an idea or plan which you keep secret and can use if you need to (especially in order to gain an advantage over somebody): They think they’ve won the contract but we’ve still got a couple of aces up our sleeve.
See also: ace, have, sleeve, trick, up