A Cantonese Waterslide is a sex move, where you have sex with a Cantonese transvestite while she is dressed as Meg from Family Guy, just before climax, flip her onto her back and pee in sync with her. Then ejaculate onto the flow of piss, causing the semen to flow down her back like a water slide.
"Hey why were you so late to the party"
"I was giving my girl a Cantonese Waterslide while listening to Nettspend, It was awesome!"