The tan one receives by commonly wearing ridiculously long shorts in the summer. Generally only males are rendered with this strange positioning of tan lines, primarily due to the popularity of basketball shorts which go down to only about six inches from the ground. Since only about half of the calf is exposed to sunlight it's the only part of the leg which becomes tan. When worn in conjunction with equally popular (and equally bogus) over-sized shirts, the resultant tan leaves the appearance of a pasty-white scuba suit worn by the afflicted subject.
Jill and Sally were at the local watering hole.
Jill giggled. "My boyfriend John always wears basketball shorts in the summer, so every fall he winds up with a scubatan."