摘要：You eat her food, enjoy her hospitality and then criticize her behind her back. （你吃她的东西，享受她的款待，然后在背后批评她。） [柯林斯高级学者辞典]

behind somebody's back的字面义是“在某人后面”，引申为“没有某人的同意或知晓”，如

【例句1】They went ahead and sold it behind my back. (没有我的同意，他们直接过去把它卖了。)[牛津高级学者辞典]

【例句2】I should have realized that he'd go behind my back (=do something without telling me). (我应该意识到他会在我背后做某事而不告诉我。) [朗文当代英语词典]

【例句3】Have you been talking about me behind my back? (你一直在背后议论我吗？)[牛津高级学者辞典]

【例句4】I don't like the idea of the two of them talking about me behind my back. (我不喜欢他们俩在背后议论我。)[朗文当代英语词典]

【例句5】You eat her food, enjoy her hospitality and then criticize her behind her back. （你吃她的东西，享受她的款待，然后在背后批评她。） [柯林斯高级学者辞典]

【例句6】I dread to think what they say about me behind my back. (我不敢去想他们在背后说我什么。)[剑桥高级学者辞典]

“当某人的面”是to somebody's face，如

【例句7】People say bad things about him behind his back, but never to his face. （人们在背后说他坏话，但从不当着面说。）[牛津高级学者辞典]

【例句8】If you've got something to say, say it to my face.（如果你有话要说，当着我的面说。） [剑桥高级学者辞典]

【例句9】He's a liar, and I've told him so to his face many times.(他是个骗子，我已经当面告诉过他很多次了。) [牛津高级学者辞典]

【例句10】I told him to his face just what I thought of him.（我当面告诉他我对他的看法。） [朗文当代英语词典]

【例句11】Her opponent called her a liar to her face. （她的对手当面说她是个骗子。） [柯林斯高级学者辞典]

"当着面”介词是to，因为有凑近脸的感觉；“背着”介词是behind, 其be-有“在......附近”的静处空间感。

来源：三秋听雨