An "ace up sleeve" refers to a hidden plan or an unexpected advantage that can be used when needed. However, the opposite of having an ace up one's sleeve is being caught unprepared or at a disadvantage. This can mean not having a backup plan, being unable to adapt to changes, or lacking the necessary resources to succeed. Other antonyms for "ace up sleeve" can include transparency, honesty, and playing by the rules. To avoid being caught without a plan, it's important to stay organized and prepared, and to have contingency plans in place for unforeseen circumstances. This way, you can stay ahead of the game and avoid being caught off guard.