A spoilsport is someone who ruins a game for everybody else. Don't be a spoilsport, the spoilsport stopped it, called him a spoilsport.
It is a mild insult and usually aimed at somebody enforcing a rule that everybody else was happily ignoring — which means being called one is often a sign of being right. Note that the compound is spoil plus sport, where sport means fun rather than athletics, as it did when the word was made.
1nounspoilsportSomeone who ruins a game for everybody else.
“Don't be such a spoilsport — come and dance with us!”
spoilsport is a person who ruins fun.
From the phrase spoil sport, where sport means fun or amusement rather than athletics.
From the verbal expression spoil sport (“ruin the amusement”).
Etymology from Wiktionary, CC BY-SA
Practised in Games & Toys