A wet blanket is somebody who spoils other people's fun. Do not be a wet blanket, a bit of a wet blanket, the wet blanket of the group.
The image is exact, since a wet blanket is what you smother a fire with. It is mild as insults go and is usually said to somebody's face rather than behind their back.
1nounwet blanketSomebody who spoils other people's fun.
“Don't be such a wet blanket; come to the party and have some fun!”
wet blanket is a person.
From the wet blanket used to smother a fire.
Suggesting that such a person would extinguish fun in the same manner a wet blanket can smother and extinguish a fire.
Etymology from Wiktionary, CC BY-SA
Practised in Idioms