E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
A low, vulgar fellow; an omnibus conductor. Either from cadet, or a contraction of cadger (a packman). The etymology of cad, a cadendo, is only a pun. N.B.The Scotch cadie or cawdie (a little servant, or errand-boy, or carrier of a sedan-chair), without the diminutive, offers a plausible suggestion.
All Edinburgh men and boys know that when sedan-chairs were discontinued, the old cadies sank into ruinous poverty, and became synonymous with roughs. The word was brought to London by James Hannay, who frequently used it.M. Pringle.