The idiom "ace in the hole" originates from the game of poker, where players are dealt a hand of cards, and one card (the "hole card") is dealt face-down and kept secret until the end of the game. The ace is typically the highest-ranking card in many variations of poker. Therefore, this idiomatic phrase is used  to describe a secret advantage, resource, or strategy that someone keeps hidden and reserves for use in a pivotal or unforeseen circumstance.

What is the origin of the idiom "ace in the hole" and when to use it?

英語での「ace in the hole」の定義と意味

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