The burning/itching feeling one gets after a wet bowel movement. Usually the effect of not wiping ones rectum well enough. other symptoms include awkward way of walking, soggy undergarments and a brownish stain on sed underpants. the only hope of ridding yourself of rainbows is using moist tissue paper to re-wipe your rectum. Symptoms will dissapate rapidly.
Mike- "Dude why do you keep scratching you butt-crack?"
Josh- "I just took a shit man, these rainbows are killin' me!"
Mike- "Dude just wipe your ass better then it'll go away."