wet blanket

n. Informal
One that discourages enjoyment or enthusiasm.

A person who discourages enjoyment or enthusiasm, as in Don't be such a wet blanket--the carnival will be fun! This expression alludes to smothering a fire with a wet blanket. [Early 1800s]

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The noun has one meaning:

Meaning #1: (informal) someone who spoils the pleasure of others
  Synonyms: spoilsport, killjoy, party pooper

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