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.
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 entry for self-defence (packet etymology_text: From self- + defence.)
Two branches
當下保護自己這件事
這一支就是畫面本身：手臂舉起來的那一刻。它回答的是「你為什麼這樣做」，所以特別容易出現在法庭和新聞裡。
noun · B2
The act of protecting yourself, the people with you, or your property, when someone attacks you or threatens you. The word is used a lot in law, to say that a person only acted because they had to stop an attack.
自衛
事先學起來的本事
同一個舉手擋下的畫面，只是時間往前挪：還沒發生，你先去學怎麼擋。於是它從一件事變成一套可以上課、可以練的技術。
noun · B1
The skills and moves you learn so that you can protect yourself if someone attacks you. This is what people go to a self-defence class to study.
防身術