Slap-Happy: The names are changed, the story is true
When we were in high school, my friends and I liked to hang out at my friend Wally's house because his parents both worked, which meant we could be as raunchy as we pleased. The only girl who ever showed up was Wally's younger sister, Wendy, who was a freshman.
One afternoon, we were sitting around Wally's living room telling stories about our sexual exploits (most of which I'm sure were made up), when a kid named Keith stood up and whipped out his erect cock and said, 'Look what you jackoffs did to me!' Wendy, who'd been sitting right next to Keith, jumped up and said, 'BAD boy!' and slapped Keith's penis.
Keith flinched but stood his ground. His erection swayed and bounced from the force of Wendy's slap. Wendy thought this was so funny, that she slapped it again. 'BAD boy!' This sent Keith's erection into such a frenzy of gyrations that everyone, including Wendy, started laughing. Wendy slapped Keith's boner again and again, each time shouting, 'BAD boy!'
After the tenth or eleventh slap, the predictable happened. A long rope of cum spurted furiously out of Keith's cock, hitting Wendy right in the sweater. She screamed as Keith, moaning and gasping, took his spasming cock in his hand and finished the job. Wendy ran shrieking from the room, and didn't come back. We made Keith clean up the mess.
(Footnote: I tried the 'slap-happy' method on myself a few times. It worked, but not very well. I think it would have been more fun if Wendy was doing the slapping.)