One way to hold this word: a wall that cannot be taken. Attackers arrive, they push, they wait, and the place simply does not open. The word says nothing about who is inside or whether they deserve to be safe — only that whatever is pushing from outside is not getting in.
可以想成一面「拿不下來的牆」。敵人來了，推了，圍了很久，那個地方就是不開。畫面的重點只有一個方向：外面用力，裡面不動。這個字不管裡面是誰，也不管他們該不該平安，它只說一件事——從外面來的力量進不去。把這面牆換成看不見的東西，就得到它第二個用法。
From the word’s history
From Old French imprenable: im- ('not') plus prenable ('takable'), from prendre ('to take'), which goes back to Latin prehendere ('to seize, to grasp'). A town that falls is said to be taken, so 'not takable' is exactly 'cannot be captured'.
來自古法文 imprenable：im-（不）加上 prenable（拿得下的），而 prendre（拿）再往前是拉丁文 prehendere（抓住、奪取）。城被攻陷，英文正好說 be taken（被拿下）。所以「拿不下來的」就是「攻不破的」，字面和今天的意思幾乎是同一句話。
Wiktionary, "impregnable" (etymology 1)
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