Catch somebody with their pants down
Definition 1 of 1
Pro Tip 1/3Vulnerability Emphasis
Emphasizes that someone was unprepared and vulnerable to unexpected problems or revelations. 
The sudden inspection caught the staff with their pants down.
Pro Tip 2/3Metaphorically Speaking
The phrase is used figuratively, not literally; it's about being caught unprepared, not without pants. 
His sudden question about finances caught me with my pants down.
Pro Tip 3/3Idiomatic Nature
Remember it's an idiom; it won't make sense to someone if translated word-for-word into another language. 
To a non-English speaker, 'caught with their pants down' was a confusing phrase.