A sexual act where the male dips his penis first into the vagina, and then tactfully slides it into her anus so as to casually transition from vaginal to anal sex. It generally causes great discomfort and so the transition is often short-lived, unless the girl happens to like it.
Bro 1: "Yo I gave Emily the Dip N' Slide, she reacted violently."
Bro 2: "Thats awesome, I tried it and Hillary loved the Dip N' Slide! But I'm not surprised her butthole could take it."