Relationships between farm equipment mechanics and their customers are different from those between urban auto mechanics and their customers. I'm pretty sure auto mechanics working in big cities don't greet customers by saying, " 'Morning, Grubbie!" or "How ya doin' Big Al? or "How's your day going, Mouse?" Sometimes I know the origin of customers' nicknames; sometimes I frankly don't know them by any other name.
Grubbie is easy---his last name is Grubbs. Big Al's personality and physique matches his name. I knew Mouse for 20 years before I cranked up nerve to ask how he got the nickname, and he explained that as a baby and child he was so small and scrawny that somebody started calling him "Mouse," and it stuck.
My cousin Dave, a farmer and livestock trucker, and I were talking about rural nicknames, and he quickly tossed out a long list he's run across in his travels. "Dote" is a local personality and nobody, including Dote, knows why we call him Dote. The same goes for Buck. The Sparky I know has the last name Sparks, but the Sparky that Dave knows is an electrician. We grew up with a neighbor named Spitz, and didn't know until he died--via his obituary--that his given name was Earnest. Dave says he knows a guy whose real name is Gary, but he's listed in the local phone book by what everybody calls him: Rooster.
Personally, the other mechanics at our dealership call me by several nicknames, including Dan-o, Danimal, or Dan'l. Customers generally call me Dan, though I've heard through the grapevine that a few don't actually know my first name, and generically refer to me as, "that #^%! Anderson."