When one peels back their foreskin, and put skittles inside of it before ejaculating. Preferably three, and has someone suck them out and states "Taste the Rainbow"
"Ah yeah baby, suck it good, I bet you love tasting the rainbow"
A common phrase on sites such as Derpibooru, e621, Rainbooru and Rule 34 that means wanting to impregnate the character Rainbow Dash from the series My Little Pony Friendship Is Magic.
David: dude, I want to cum inside Rainbow Dash
Chad: fucking loserlmao