Wet blanket.
[Idiom] WET BLANKET. - Brighten the Vibe by Understanding This Expression
Someone who dampens the mood.
A "wet blanket" is a person who dampens spirits or lessens the enthusiasm within a group. Imagine someone throwing a wet blanket over a warm, cozy fire – the fire is your fun, and the wet blanket is the person who’s bringing everyone down.
Example Sentences
Don't be such a wet blanket, join the fun!
Don't spoil the fun, join in!
He was known as a wet blanket at parties because he never participated and complained a lot.
He was known as a party pooper because he never participated and complained a lot.
Whenever they planned something adventurous, she would be a wet blanket, worrying about all the things that could go wrong.
Whenever they planned something adventurous, she would worry and spoil the fun, thinking about what could go wrong.