Definition of "off his guard" []

• Not prepared or vigilant (adjective)

WordNet. Princeton University. 2010.

• "In licensing the new Divorce Tract, even though it did consist mainly of extracts from Bucer, Mr. Downham must have been either off his guard or very good-natured."
• "Taken off his guard and unconscious of the irony, Walewski made the admission that the three hundred were debarred from the pleasure of paying him a visit because, though pardoned, they were not released!"
• "At Oakland, one day, the indefatigable and irresistible "Aunt Susan" caught him off his guard by persuading his daughter, Kitty Reed, who was his idol, to ask him to say just one word in favor of our amendment."