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.
From the word’s history
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)
登入就可以看每一支怎麼連回核心畫面。