Latin for 'other me', and the phrase has never really left that picture. Imagine a second you standing beside you: same thoughts, same loyalties, but a different face and a different name. English uses that picture in two directions. Point it inward and the second you is a costume — a stage name, a pen name, a secret identity, the loud version of a quiet person. Point it outward and the second you is another human being who is so close, and thinks so much like you, that they can answer in your place. Both keep the same test: is this, in some real sense, still me?
Wiktionary gives the phrase as Latin alter ego, literally 'other me'. Both pieces are still doing that work in modern English, which is why the core image is simply the literal reading.
來源字典寫明這是拉丁文的 alter ego，字面就是「另一個我」。兩個零件在現代英文裡還在做同樣的工作。所以核心畫面其實就是字面意思本身。
Wiktionary (kaikki extract), entry 'alter ego', etymology_text: From Latin alter ego (literally "other me").
Two branches
自己造出來的另一個自己
往內指的那個方向：第二個你是你自己造的，是一套可以穿上、可以脫掉的身分。
noun · C1
An invented second self: a character or name that someone uses to show a side of themselves that is very different from the way they usually are. A writer's pen name, a singer's stage character and a hero's secret identity can all be described this way.
另一個自我、分身
像第二個你的那個人
往外指的那個方向：第二個你不是戲服，而是另一個真人。這一步靠的是「想法一樣」，所以他開口等於你開口。
noun · C1
A person you trust completely, who thinks the way you do and can speak or act for you, so that having them there is almost like having a second version of yourself.
第二個自己、知己