catch with 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.
See also: catch, down, pant
Farlex Dictionary of Idioms.

catch (one) with one's pants down

Fig. to discover someone in the act of doing something that is normally private or hidden. (Literal uses are possible.) Some council members were using tax money as their own. But the press caught them with their pants down and now the district attorney will press charges.
See also: catch, down, pant
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs.

catch (someone) with his pants down, to

To embarrass someone, to surprise someone when he is at a disadvantage. The term is American and there are several theories about its origin. One holds that it comes from a husband catching another man with his wife. Another claims it refers to an enemy catching a soldier relieving himself.
See also: catch, pant
The Dictionary of Clichés by Christine Ammer
See also:
  • catch (one) with (one's) pants down
  • catch somebody with their pants down
  • catch someone with their pants down
  • catching
  • catch-as-catch-can
  • catch as catch can
  • like a greased pig
  • catch (a)hold of (someone or something)
  • catch hold of
  • catch/get/grab/take hold of somebody/something