E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
The French have a good slang term for these conveyances. They call an omnibus a Four Banal (parish oven).
Of course, omnibus (for all) is the oblique case of omnes (all). Yet Howitt, in his Visits to Remarkable Places (1840), says Cabs and cars and omnibi and stages (p. 200). The plural of omnibus is omnibuses.