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Phrase(s): have something up one’s sleeve [and] have an ace up one’s sleeve
Fig. to have a secret or surprise plan or solution (to a problem). (Alludes to cheating at cards by having a card hidden in one’s sleeve.) • I’ve got something up my sleeve, and it should solve all your problems. I’ll tell you what it is after I’m elected. • The manager has an ace up her sleeve. She’ll surprise us with it later.
McGrawhill's American Idioms And Phrasal Verbs