If you say that someone is a wet blanket, you are criticizing them because they refuse to join other people in an enjoyable activity or because they want to stop other people enjoying themselves.
[informal, disapproval]
wet blanket in British English
noun
informal
a person whose low spirits or lack of enthusiasm have a depressing effect on others
wet blanket in American English
a person or thing whose presence or influence lessens the enthusiasm or joy of others
Examples of 'wet blanket' in a sentence
wet blanket
Once, and once only, I thought I saw something -- a darker shadow, high up like a mountain thrusting the wet blanket of the fog skyward.
Innes, Hammond HIGH STAND (2002)
He's a bit pompous for going out with --- he can be a right wet blanket.
Jane Asher LOSING IT (2002)
spluttered Sam when he finally got free of the wet blanket.
Garth Nix LIRAEL: DAUGHTER OF THE CLAYR (2002)
Alfredo Mancuso and Gino Laveri are on my back, and the police are all over me like a wet blanket.