[ace in the hole] {n. phr.}
1. An ace given to a player face down so that other players in a card game cannot see it.
* /When the cowboy bet all his money in the poker game he did not know that the gambler had an ace in the hole and would win it from him./
2. {informal} Someone or something important that is kept as a surprise until the right time so as to bring victory or success.
* /The football team has a new play that they are keeping as an ace in the hole for the big game./* /The lawyer's ace in the hole was a secret witness who saw the accident./
Compare: CARD UP ONE'S SLEEVE.