from The American Heritage® Dictionary of the English Language, 4th Edition
- n. One who mars the pleasure of others.
from Wiktionary, Creative Commons Attribution/Share-Alike License
- n. Someone who puts an end to other's fun, especially harmless fun.
from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.
- n. someone who spoils the pleasure of others
From the verbal expression spoil sport ("ruin the amusement"). (Wiktionary)