Ace in the hole

This originates from the game of poker, where a card dealt face down and kept hidden is called a ‘hole card’, the most propitious card being the ace.

The earliest printed citation I can find is from the Iowa newspaper The New Era, June 1886:

“Thus matters went on until four cards lay in front of each man, face up and one turned down. Not a pair in sight and everyone thought each man had an ace ‘in the hole’.”

The term was used as the title of a 1951 film, starring Kirk Douglas. The plot involved Douglas’s character delaying a rescue in order to cash in on the notoriety of having exclusive reporting rights to the rescue attempt.