Card up sleeve is an idiomatic expression that refers to a secret advantage, skill, or tactic that one possesses and can use to gain an advantage over others. There are several synonyms for this expression, including "ace in the hole," "trump card," "hidden agenda," "secret weapon," "surprise attack," "sneak attack," "stealth move," and "underhanded tactic." These colloquial phrases represent a variety of strategies people can use to gain an advantage or achieve their goals. Regardless of which phrase you choose to use, it's essential to remember that relying solely on tricks and gimmicks is not a sustainable way to accomplish long-term success or happiness.