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A hidden advantage or resource kept in reserve until needed: “The coach was certain that his new trick play would turn out to be his ace in the hole.” This term comes from the game of stud poker, in which one or more cards are turned face down, or “in the hole,” as bets are placed. The ace is the card with the highest value.
Something held privately in reserve until needed, esp for a winning stroke; a hidden reserve or advantage
[fr poker term in the hole for a card dealt face down in a stud game]