It's from the french - char à bancs: “wagon with benches”, originally a long, four-wheeled carriage with several rows of forward-facing seats, originated in France in the early 19th century. It was horse-drawn and was used by private owners to convey guests on excursions.

The term was also later applied to motor charabancs - large usually open-top vehicles based on large car or light commercial chassis, again, used for excursions rather than for stopping routes (for which you'd use a motor-bus)

See here for a picture.

Best regards,