In rural Kentucky, after men went off to war and died in the 1940's, mothers needed to reproduce and so they had sex with their own sons, having inbreed children. These inbred children had multiple defects, and earned the nickname "Yellow Hammer"
Here's an incident that got me sent to the slammer; I'm in the club rappin to this yellow hammer. This bitch was holdin', severely cut; so I'm rappin to her right, to see if she'll fuck.
A person of the trades (auto mechanic, carpenter, plumber) who knows virtually everything about his particular line of work, but little to nothing about anything else. This can include life itself. While not considered a derogatory term by any means, it *implies* said person would do well my expanding their horizons.
Actually, it's a reference to inbred people from Wilmington, IL. Though being that Alabama is in the South and most ignorant people think inbreeding is a southern only thing, I can see how some could think that.
Wilmington became famous for being a stop on U.S. Route 66, which followed the route of modern-day Illinois Route 53. A notable attraction for travelers along this route is the hammers or yellow hammers that live in brodies woods they once worked on the railroad using yellow hammers and set up residency in Wilmington. They bred within the family and became mutant from insest.