It's the sound that a man's balls make when he's fucking her from behind.
Will pulled a happy-slappy on Lainey.
by James Lambert March 18, 2007
1 more definition
Criminal acts, often caught on mobile video phone's and passed around to other children to witness. Often these are scenes of bullying, rape, and, in some cases murder.
Newspaper headline: "Happy slappy incident: gang of 19, average age 12, gang rape girl, 13".
by Flatster November 24, 2005