Wet blanket
The phrase "wet blanket" typically refers to a person who discourages or dampens the enthusiasm, excitement, or enjoyment of others by being negative, critical, or unenthusiastic. It is often used metaphorically to describe someone who puts a damper on the mood, fun, or excitement in a given situation.
An idiom is a phrase or expression that usually presents a figurative, non-literal meaning attached to the phrase. Does that sound fun? It can be!
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