他从背后悄悄逼近了那毫无戒备的受害者。

He had crept up on his unsuspecting victim from behind.

《牛津词典》

他两次目睹群豹向毫无戒备的小狒狒猛扑过去。

Twice he had seen leopards pouncing on unsuspecting young baboons.

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当你第一次与他对视，你发现他眼里的胆怯，但当你与他接触，他又不禁显露出毫无戒备的纯真。

When it's you first time to make eyes contact with him, you will find that his eyes are full of timidity. However, when you access him, he can't help showing innocence without any prevention.

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