"Card one's sleeve" is a phrase used to describe a secret plan or strategy that a person has hidden and can use to gain an advantage in a tricky situation. Synonyms for this phrase include "secret weapon," "ace in the hole," "trump card," and "hidden agenda." The term "secret weapon" emphasizes that the person has a unique advantage that no one else knows about. "Ace in the hole" has a similar connotation, but it also implies that the person is holding onto this "card" for a specific moment. "Trump card" comes from the world of card games and means a card that outranks all the others. "Hidden agenda" is more general and implies that the person has an ulterior motive that they're keeping hidden.
