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Someone who ruins other people's fun — by refusing to join in, by stopping the game, by pointing out the rule everyone was happily ignoring. It is almost always said to the person's face, lightly: 'oh, don't be a spoilsport.' That is the shape you will meet it in most often. The feeling is complaint rather than anger, and it is a bit childish, which is part of the point — it belongs to games and parties, not to serious matters. Nobody calls a criminal a spoilsport. Here is the interesting part: the person called a spoilsport is often right. They are the one saying it is late, or that this is dangerous, or that the rules say otherwise. The word is what the group uses to push back against being sensible, so it carries a hint that the speaker knows they are in the wrong and is enjoying themselves anyway.
掃興的人、破壞大家興致的人（口語，常說「別這麼掃興」）
組成spoil (to ruin) + sport (fun, amusement — the older meaning of the word, not the meaning 'a game like basketball'). So: someone who ruins the fun. The phrase spoil sport existed first, and English squeezed the two words into one name.
原始詞典資料2 筆記錄
以下是外部詞典記錄的用法，原文照列，我們還沒有寫成教學內容。上面的說明才是我們自己寫的；有些用法可能上面已經講過了。
spoilsportnoun/ˈspɔɪl.spɔːt/
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Someone who puts an end to others' fun, especially harmless fun.
- I know I am a sad spoilsport, but it would make me wretched.
- He looks a spoilsport. There are men in whose presence it is impossible to have any fun: men who are a sort of walking conscience.
spoilsportadj/ˈspɔɪl.spɔːt/
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Like a spoilsport.
- It may seem somewhat spoilsport to focus in on this negative after another important victory but to ignore it would be delusion. Southgate will not be letting it escape his attention.
資料來源：英文維基詞典 English Wiktionary（CC BY-SA 4.0）。 原文照列，未經改寫。