How to Say "wet blanket" in Spanish
The Spanish word for “wet blanket” is “ceniza” — C1 level.
English → SpanishC1
nounC1informal
a person who discourages enthusiasm

Examples
No seas tan ceniza, ¡va a ser una fiesta genial!
Don't be such a killjoy, it's going to be a great party!
Él es un ceniza, siempre piensa que algo malo va a pasar.
He's a jinx; he always thinks something bad is going to happen.
Using it as a label
When calling someone a 'ceniza,' it functions as a noun. You use it with 'ser' (to be) to describe their nature.
Don't change the ending
Mistake: “Él es un cenizo.”
Correction: Él es un ceniza. (While some regions might use 'cenizo', 'ceniza' is the more traditional form for this specific slang meaning regardless of the person's gender.)
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