https://en.wikipedia.org/wiki/Nostal...,_1955_to_1985

These cars, many street-driven to the strip and then (with good fortune) back home after the race, were grouped in fast-to-slow alphabetical classes (A/S, B/S, C/S, etc.) determined by horsepower-to-weight factors in wide ranges.

In 1960, optional classes (A/SA, B/SA, C/SA, etc.) for cars equipped with automatic transmissions were added.
This evolution led to the establishment by NHRA of the Optional Super Stock (O/SS, 1961), Super Super Stock (SS/S, 1962) and Factory Experimental (FX, also 1962) categories, followed by Modified Production (MP, 1963), in an effort to keep pace with the fast-moving technology.

Also in 1963, in cooperation with other governing national and international authorities, NHRA limited maximum engine size to 427 cubic inches (7 liters) and in 1964, started using its own horsepower factors, mainly to correct notoriously low ratings from the manufacturers.

By 1965, some of these factory and privateer purpose-built FX vehicles, like the long-established ‘Gasser’ and ‘Altered’ classes, featured changed wheelbases, fuel injection and forced air induction (blowers) - configurations not even an infinite amount of rationalization could justify as ‘Stock’ or even ‘Super Stock.’

In 1966, NHRA put the increasingly radical FX cars – some now fueled with nitromethane - into their own eliminator, officially called ‘Experimental Stock.’ But popularly they had been called ‘Funny Cars’ for some time, reflecting a common comment, “That car sure looks funny.”