When you shit yourself while riding your bicycle, and said shit hits your back tire then sprays upwards leaving a rooster tail pattern of shit up your back.
I had a greasy breakfast before that long bike ride which led to a Pooster-Tail. I feel bad for the guy behind me...
A sunburn or suntan focused on the left arm. It is caused by the left side of the body being the only half that gets exposed to sunlight while driving. Presumably, it would be located on the right side of the body in countries where they drive on the left side of the road.