Having a secret plan, idea etc.
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I think Cathy has something in her mind and she is up to something. She has been acting weird all day. She must have something up her sleeves.
have something up your sleeveinformal to have a secret plan or idea that you are going to use later Don’t worry. He still has a few tricks up his sleeve.
Cambridge Advanced Learner's Dictionary - 4th Edition
have sth up your sleeve informal
C2 to have secret plans or ideas:
If I know Mark he'll have one or two tricks up his sleeve.
© Cambridge University Press 2013
have sth up one's sleeve
If you have something up your sleeve, you have an idea or plan which you have not toldanyone about. You can also say that someone has an ace, card, or trick up their sleeve.
He wondered what Shearson had up his sleeve.
I'd been doing some quiet investigating in the meantime and had an ace up my sleeve.
up one's sleeve
: held secretly in reservehas a few tricks up her sleeve