Draw a line around a group. Inside the line people share a history, habits, jokes and information, and they know who else is inside. An outsider is a person standing on the other side of that line. What changes from sense to sense is only who drew the line: sometimes people's feelings draw it, sometimes a payroll or a membership list draws it, and sometimes it is drawn by what everyone expects before a race starts.
The source traces the word to out + side + -er: the one on the outer side. The -er ending names the person in that position, which is exactly what every modern use does.
字源資料把這個字拆成 out＋side＋-er，也就是「在外面那一側的人」。-er 這個字尾負責指出「處在那個位置的人」，而這正是它今天每一個用法在做的事。
Wiktionary etymology tree supplied in this packet (kaikki:3f6f69a190036ae4, outsider: out + side + -er)
Two branches
圈子外面的人
這一支是核心畫面直接落在人群上：線的裡面是自己人，這個人在線外。差別只在線是感覺畫的，還是名冊畫的。
noun · B1
Someone who is not treated as one of a particular group of people, so they stay on the edge of it and do not share the history, the habits or the jokes that the members share.
局外人；外人
賽前被放在圈外的人
還是同一條線，只是這次由「大家的預期」在比賽開始之前先畫好：被看好的那幾個在裡面，其他人都在外面。
noun · B2
A person, team or animal in a race or competition that most people do not expect to win.
不被看好的參賽者