a wet blanket someone who has a depressing or discouraging effect on others.
☞ A dampened blanket can be used to smother a fire; the image here is of a person extinguishing a lively or optimistic mood by their gloominess or negativity.
2012Guardian How does an actor known for her versatility … embody the grown-up girlfriend who gets in between the boy and his bear without seeming like the wet blanket who's spoiling the party.