From Longman Dictionary of Contemporary Englishspoilsportspoil‧sport /ˈspɔɪlspɔːt $ -spɔːrt/ noun [countable] informalSPOILTAKE PART/BE INVOLVEDsomeone who spoils other people’s funDon’t be such a spoilsport.
Examples from the Corpus
spoilsport• Don't be a spoilsport, Richard. Come play with us.• I decline, and feel like a spoilsport.• "I don't think we should go in there - someone might see us." "Oh, don't be such a spoilsport - come on!"• And last night they were brandedspoilsports after claiming not enough viewers are interested in the national game.• Civicspoilsports, however, banned the annualnudeswim off Bondi Beach.• Until some spoilsport thought to measure the 100-yardtrack - and found it was just over 70 yards.