You can think of a face you are sure you know. Because you know it, you relax, you stop checking, you let it come close. And then it turns out not to be what it looked like. Both meanings live in that gap between the familiar surface and what is actually underneath, and in both of them the harm is done by your own trust, not by anything obvious.
From the word’s history
The phrase is a calque of French faux-ami, shortened from faux amis du traducteur, 'false friends of the translator', first used by Maxime Kœssler and Jules Derocquigny in their 1928 book Les Faux Amis. The naming itself is the image: a word that wears the face of a friend and then misleads you.
這個說法是從法文 faux-ami 直譯過來的，完整原話是 faux amis du traducteur（譯者的假朋友），出自 Kœssler 與 Derocquigny 1928 年的書 Les Faux Amis。命名本身就是畫面：一個戴著朋友面孔的字，等你信了才把你帶偏。
Wiktionary, "false friend"
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