One way to hold this word: someone puts their arms up to stop a blow. Everything in the word is in that picture. The arms are up to stop something, not to start it — defence, not attack. And they are your own arms, protecting you — self. From there the word goes two ways. Sometimes it names the moment: that is why I did it, I had to stop him. Sometimes it names the preparation: this is what I learned, so that I would know what to do.
可以想成一個人把手臂舉起來擋。這個字的東西全在這個畫面裡。手臂是舉起來擋的，不是先出手的——所以是 defence，不是 attack。而擋的是自己，所以前面加了 self。從這裡它分成兩條路：一條指那個當下——「我這麼做是因為我得擋下來」；一條指事先的準備——「我學過，所以我知道怎麼辦」。
From the word’s history
The word is built from self- plus defence, so it says plainly what it means: defence of yourself.
這個字是 self- 加上 defence 組起來的，字面就已經把意思說完了：守住自己。英文裡一整排 self- 開頭的字都走這個模式，像 self-control、self-study。
Wiktionary, "self-defence"
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