...origins of the term....
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When you work the fields or construction, you get sunburned/tanned on the back of your neck.
We also call it a farmer’s tan. Take off the shirt and everything else is the color of milk.
Yep, I always heard that it described the farmers on Sand Mountain in Alabama. I saw a huge Indian Motorcycle in front off a small country store on Sand Mountain when my family was visiting relatives there. I was very young in the 1950's.