The common usage is all manner of vehicles that look something like a Cobra AC Cars produced for pure street use (not with the racing wing modifications they did, Shelby's works did, or anybody else for that matter). The usage is comparing any car body shape from any maker at any time to a new in 1963-65 street Cobra; stock street Cobra shape to one with any kind of modified wings to cover larger than stock fitment wheels and tires.
It was the modern look alike car fans that took the term into their main stream cross talk and continue to use and spread the term. Most of the appellations used these days were created by individuals long after the cars were new. A hypothetical conversation might start with something like this, '(insert the name of any builder involved) is getting set up to add slabside bodies to their production'. Another variation is something like; person one, '(I just got a quote on a car from (insert the name of one of several builders), person two, 'an FIA?' (another grossly misused term as used in modern times), person one, 'no a slabside'.
In any case, no matter when the first person coined the term, the term as used seems as inappropriate as calling any car with a fiber and resin composite chassis and or body 'a plastic pig', a car with an aluminum chassis and or body 'a beer can', or any car sold as components or owner fabricated a 'kit car'.
Dan
