noun
a person who spoils the enjoyment of others, typically by being disapproving or overly serious
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The teacher was seen as a killjoy by the students because she always enforced strict rules.
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Don't be such a killjoy and come join us for the party!
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She's such a buzzkill, always being a killjoy at every gathering.
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His negative attitude was like a killjoy, sucking the energy out of the room.