party-pooper

/P AA1 R T IY0 P UW1 P ER0/
noun
  1. 1

    One who dampens fun; especially, one who declines to participate in or enjoy a party or some other group activity; one who does not agree with the opinions of a group.

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Source: https://en.wiktionary.org/wiki/party%20pooper, https://en.wiktionary.org/wiki/party-pooper