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?
From the word’s history
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, "alter ego"