Think of the person on a stage who has been put there for someone else's benefit. He is not there to shine; he is there so that the other performer shines. He takes the joke, asks the flat question, or plays the part the act needs. Now take that off the stage: a person placed where someone else needs them, doing what that person needs done.
From the word’s history
The source says the word may be a shortening of Russian студент (student), and that its earliest meaning was a stage assistant — the actor who helps a comedian. It may have come from Yiddish vaudeville slang. That earliest meaning is exactly the stage picture the other uses grow out of.
來源說這個字可能是俄文 студент（學生）的縮寫，最早的意思是「台上的助手、幫喜劇演員的演員」，也可能來自意第緒語的雜耍圈用語。來源自己用的是「可能」，所以這條線索只能當成參考。不過它和今天的核心畫面對得上：一開始就是站在旁邊、替主角服務的那個人。
Wiktionary (kaikki extract, source_ref kaikki:3f6f69a190036ae4:624998996:24026)
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