I think Bill has it right, intermediate body (light weight) together with large c.i. motor. I remember the term first coming about to describe the GTO and expanding when competing companies came out with their versions (GM A-bodies, Ford Fairlanes and Torinos, Roadrunners, AMC Hornets, etc.). I've also heard Duesenbergs mentioned, as well as the '50 Olds and some '30s Lincolns. All depends on your point of view I guess.