Booty Sweat Rash
Usually referred to by it's acronym B.S.R. A rash that develops, primarily in the South during the summer months, from an unfortunate combination of a poor or missed ass wipe and a prodigious ass sweat. Especially common among outdoor laborers and cyclists.
John never got a chance to do a proper clean-up after he sharted, so by the end of the ride his booty sweat rash was raging.