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.
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.