a person who hasn't the interest or vitality to participate actively in a social party and whose mood, attitude, or personality lessens others' enjoyment; killjoy.
a person whose behaviour or personality spoils other people's enjoyment
[C20: originally US]
Collins English Dictionary - Complete & Unabridged 10th Edition
2009 © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009
Don't invite Martha. She's such a party-pooper.]]>
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Dictionary of American Slang and Colloquial Expressions
Dictionary of American Slang and Colloquial Expressions by Richard A. Spears.