Ace Up the Sleeve
An ace up the sleeve refers literally to a concealed card used to cheat, and figuratively to a hidden advantage.
The simple explanation
Taken literally, the phrase describes keeping an extra ace hidden outside the normal deal so it can be used dishonestly. In everyday speech, it can simply mean a surprise advantage or plan. The surrounding conversation tells you whether someone means actual cheating or is speaking figuratively.
A closer look
A privately dealt ace is a normal hole card; an ace secretly introduced from outside the deal is cheating. This is the distinction between the literal meanings of "ace in the hole" and "ace up the sleeve." Figurative use can be innocent: a player might describe an unexpected adjustment as their ace up the sleeve. The expression alone does not establish that anyone broke the rules.
A practical example
A player says, "I still have an ace up my sleeve," meaning a plan the others have not seen. They may be speaking figuratively. Actually bringing a concealed extra card into play would be cheating.
Full explanation
An ace up the sleeve refers literally to a concealed card used to cheat, and figuratively to a hidden advantage.
The literal image is a player concealing a card outside the normal deal and using it to gain an unfair advantage. That is cheating.
The expression is also used more broadly for a secret plan or resource. A figurative ace up the sleeve can be legitimate. Context matters, especially when distinguishing ordinary poker talk from an allegation of cheating.
Alternative spelling: Ace up One’s Sleeve, Ace-up-the-Sleeve