be caught with (one's) trousers down

To be taken by surprise; to be caught in a vulnerable (often embarrassing) position or situation. When the boss started firing questions at me, I was caught with my trousers down and looked like a fool. I just hadn't considered all of those potential outcomes yet and didn't know what to say. We seriously underestimated our opponents on Saturday and were caught with our trousers down. Oh man, I was totally caught with my pants down on my date tonight when my credit card was declined at the restaurant.