adjective phrase: vastly different.

This developed around the 1990's. It was a superlative created to indicate how different one thing is from another. Whereas one could say "another story altogether" or "that's a wholly different story," the term "whole nother" seemed to develop to create a superlative of "another."

Another way to say “a whole ‘nother story,” or when a new topic is on a different level or problem