The act of coitus interruptus followed by urination, i.e. pulling out early, cumming on a female's stomach, then immediately
pissing whilst the penis is still erect, such that both fluids form a rainbow-like arc.
After six King Cobras and already breaking seal, I couldn't hold my piss in anymore, so I figured I'd give my girl a double rainbow. She was so happy she cried 'Double rainbow all the way, what does this mean!?!'