n. someone who ruins a good time. (In the way that a wet blanket is used to put out a fire.) : Oh, Martin! Why do you have to be such a wet blanket?
Dictionary of American Slang and Colloquial Expressions by Richard A. Spears.Fourth Edition. Copyright 2007. Published by McGraw Hill.
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A person who discourages enjoyment or enthusiasm, as in Don't be such a wet blanketthe carnival will be fun! This expression alludes to smothering a fire with a wet blanket. [Early 1800s]