Someone (generally from the South) who would vote for a yellow dog if it was a Democrat. Alternately, someone who would rather vote for a yellow dog than a Republican. The term probably dates back to the 19th century and gained national recognition in the 1928 Presidential election. Today, used most often as a term of approval or self-identification.

When you eat dirt for the first time and leave your family to build a shack in the woods

a republican in sheeps clothing

More than likely some sort of Southern White, urban or hick, who is enamored with radical liberalism that he sees on the East and West Coasts of the USA. May be a biker, Vietnam veteran, etc. Usually champions the causes of minorities and homosexuals, but yet tries to keep the facade of being somebody that will die for Old Glory.