Good question! I had no idea and could give no answer. A quick look at wikipedia, and we learned that the word ‘bus’ is the shortened form of the Latin word ‘omnibus‘.
One etymology (the study of the origin of words) holds that “omnibus” is derived from a hatter’s shop which was situated in front of one of the first bus stations in Nantes, France in 1823. “Omnes Omnibus” was a pun on the Latin sounding name of that hatter Omnès: omnes meaning “all” and omnibus means “for all” in Latin. Nantes citizens soon gave the nickname of Omnibus to the vehicle.
When motorised transport started replacing horse-drawn transport in 1905, a motorized omnibus was called an autobus, a term still used today. Wikipedia
There you go!