Catch somebody with their pants down - Idioms by The Free Dictionary
catch somebody with their pants down
catch (one) with (one's) pants down
To encounter or catch one by surprise in a vulnerable and often embarrassing position or situation. Thanks to that anonymous tip, we were able to catch the drug smugglers with their pants down—right when they were making the exchange. Jeff was caught with his pants down when he came to the meeting without having read the brief and the boss started firing questions at him.
catch somebody with their ˈpants down (British English also catch somebody with their ˈtrousers down) (informal) find or trap somebody when they are unprepared or not paying attention: After the devastating attack on its military bases, the country was determined not to be caught with its pants down a second time.