Original by Philip Gibbs 1997.
The symbol c seems to have been used for the speed of light (in vacuum) for a long time. Einstein used other symbols such as V in his early relativity papers but most early books on relativity used c.
It probably stands for the Latin word celeritas meaning swift. Celerity is also an infrequently used word in English for speed and the same root is used in the words accelerate and decelerate.
It would be interesting to know more about who used c for the speed of light (or other speeds) before the twentieth century. If anyone knows please e-mail me at pg@pobox.com