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.