1
general::
Surprise, take unawares. This term is often used in the passive, as in The United States was really caught napping the day the Japanese bombed Pearl Harbor. It originated in the mid-1500s as to be taken napping. Also see under OFF GUARD.
American Heritage Idioms
2
general::
Phrase(s): catch someone napping [and] catch someone off balance; catch someone up short
to come upon someone who is unprepared; to surprise someone. (See also asleep at the switch.) • The enemy soldiers caught our army napping. • The thieves caught the security guard napping. • I didn’t expect you so soon. You caught me off balance. • The teacher asked a trick question and caught me up short. • The robbers caught Ann off balance and stole her purse.
McGrawhill's American Idioms And Phrasal Verbs