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1·He had crept up on his unsuspecting victim from behind.

他从背后悄悄逼近了那毫无戒备的受害者。《牛津词典》

2·A practical joke is a trick that is played on an unsuspecting victim to make him or her look silly or embarrassed.

恶作剧是发生在一个毫无戒备心的受害者身上的诡计，让他或她很尴尬或难堪。