Cab is a shortening of cabriolet, reflecting the design of the carriage. It replaced the hackney carriage as a vehicle for hire; with the introduction of clockwork mechanical taximeters to measure fares, the name became taxicab.

You’re gonna use this piece of information in a conversation some day.  Winning!

The Hansom Cab – designed and patented in 1834 by Joseph Hansom, an architect from York. Now you know.