If you have something up your sleeve, you have an idea or plan which you have not told anyone about. You can also say that someone has an ace, card, or trick up their sleeve.

e.g., He wondered what Shearson had up his sleeve. 

-I’d been doing some quiet investigating

and had an ace up my sleeve.

-I’ve got a few tricks up my sleeve

 if he decides to pursue legal action against me.

Tradução: ter algo guardado na manga, ter um truque na manga, ter um plano secreto, ter um ás na manga.