spit
spit or piss into the wedding cake {v. phr.}, {vulgar}, {avoidable}
To spoil someone's pleasure or celebration by doing or saying something harsh or unseemly in an otherwise happy gathering; bring up depressing or unhappy subjects at a supposedly happy time.
Stuart really spit into the wedding cake when he told Burt in a bragging fashion that Lucy, Burl's bride, used to be his girlfriend.
Categories:
Source: A Dictionary of American Idioms
