It is common for certain occupations to contribute terms to the broader vocabulary of society. One such term comes from the world of trucking, with good buddy becoming a fairly common phrase used to punctuate conversation, especially in the southern US.
The term first came into popular usage with the rise in popularity of CB radio. It really made the leap into the broader culture with the film Smokey and the Bandit. Bandit (played by Burt Reynolds) and Cletus (Jerry Reed's most famous film role) bantered on the CB while running a hot load of Coors beer, dodging Smokey Bear, and otherwise corrupting Sally Field. This single film arguably did more to popularize the myth of trucking, CB radio, and cop stereotypes than any other movie.
Within the trucking community itself, the phrase has had a negative connotation almost since the film came out. It is used as a term of derision in an industry which clings to its machismo image tighter than a spinster aunt clings to her virginity.
In brief and starkly vivid imagery, a good buddy is a male friend who goes out and gets a pair of blow jobs, bringing one back to share with his guy pal.