Caddie or cadet used to denote the passenger of a horse-coach picked up for personal profit by the driver (i.e placed next to the driver and not in the already booked interior). So a 'cad' is a man who picks up women, profits from them, and then leaves them by the road side. 'Caddie' or 'cad-et', as in the diminutive -- 'there goes a cad and his cadette'.
Such romantic etymology is enough to make a man want to don his oilskin and mount his horse with whip and smile at the ready.