From Sun Tzu's Art of War (孙子兵法): 攻其无备,出其不意 — attack where the enemy is unprepared, appear where you are not expected. One of the most quoted military maxims in Chinese.
grammar
Used as an adverbial modifier: 出其不意地 + verb. Can also function as a predicate: 这次行动完全是出其不意的.