A rainbow in the underwear occurs when someone (presumably a man) has made the greatest possible variety of excretions in his underpants. This would consist of a brown stain, a yellow stain, and an off-white stain.
Jean Pierre Garnier never wipes his ass, so he has skid marks; he has also been known to have wetdreams almost nightly. A few months ago thirty people came to his house to scream at him. He pissed himself and a made a rainbow in his underwear.