不備

unprepared / off guard

no; not so / (bound form) not; un-

to prepare / get ready / to provide or equip

(bound form) to prepare; to equip / (literary) fully; in every possible way

to take advantage of sb in an unguarded moment (idiom) / to take sb by surprise

to catch an enemy off guard with a surprise attack (idiom, from Sunzi's "The Art of War" 孫子兵法|孙子兵法[Sun1 zi3 Bing1 fa3])

see 出其不意，攻其不備|出其不意，攻其不备[chu1 qi2 bu4 yi4 , gong1 qi2 bu4 bei4]