Historically, the dais was a part of the floor at the end of a medieval hall, raised a step above the rest of the room. On this the lord of the manor dined with his friends at the high table, apart from the retainers and servants. In medieval halls there was generally a deep recessed bay window at one or at each end of the dais, supposed to be for retirement or greater privacy than the open hall could afford.
In the French language, the word is understood as a canopy or hanging over a seat; probably the name was an accretion from the seats of great people that were surmounted by such a feature.