I was born and raised in the South. Had several differant nick names growing up. Lurch, Slim Jim, Bean Pole, Zipper, Mule Skinner(shortened to just Skinner), Climber, Runner, Blue, Wrangler, even Bubba and many more I can not mention here. A differant name with differant groups. A nick name is assigned and you won't have anything to do with it. It will just be hung on you by someone and you won't have any way of changing it, except to get it changed to something offensive by rejecting it.
I got "Sourdough" from a lady at Holloman AFB, New Mexico. Originally it was "Sourdough Sam". When she first tagged me with the name few people knew if. She used it one day at Commanders Call, and next thing I knew people from all over the base was calling me by that name. The guys at range control shortened it and assigned "Sourdough" to me as a radio call sign.
Just after I retired my beard turned white. The name Graybeard was hung on me by a local Radio DJ. Since he called me Graybeard over the air, many people heard it and it stuck. Many people here in North Pole still call me Graybeard. I used it as well until I moved to this site, and went back to using Sourdough.