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.

Adj. Something that is expected to have a long lifespan by virtue of having existed for a long time, based on the lindy effect, named for the New York restaurant Lindy's, coined by Albert Goldman and popularized by Nassim Taleb.

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.