Catch Napping Meaning

(transitive, idiomatic) To surprise; to take advantage of the lack of watchfulness of.

Example: 1907, Harold Bindloss, chapter 7, The Dust of Conflict[1]:
  Still, a dozen men with rifles, and cartridges to match, stayed behind when they filed through a white aldea lying silent amid the cane, and the Sin Verguenza swung into slightly quicker stride. If the Colonel Morales was to be caught at all he must be caught napping, and, as they knew, he usually slept with one eye open.