Here’s a new one for those in other countries to try and match:
“Don’t be such a wet blanket” or “She threw a wet blanket on the whole project”
Now, what does it mean? LOL! A wet blanket puts out a fire and it is heavy. So it means you are a downer (Debbie downer, yet another US phrase!) or that you quashed (voided, ruined, stopped) something. Being a downer (wet blanket) also means you are very negative and are taking all the joy out of something.
Similar phrase: He put the kibosh on it. But that phrase is actually more correct and not really slang.