boredom-maker, dislike-causator
dampener (Cambridge Dictionary)
noun [ C ]
UK/ˈdæm.pən.ər/ US/ˈdæm.pən.ɚ/
something that makes something less active, less exciting, or less enjoyable:
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by Oxford Languages:
dampener
/ˈdamp(ə)nə/
noun
a thing that has a restraining or...