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

catch someone with their pants (or trousers) down

catch someone in an unprepared state or sexually compromising situation. informal
See also: catch, down, pant, someone
Farlex Partner Idioms Dictionary
See also:
  • catch (one) with (one's) pants down
  • catch somebody with their pants down
  • catch with 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