Derived from sailing ships. The 'sheet' in the phrase uses the nautical meaning of a rope that controls the trim of sail. If a sheet is loose, the sail flaps and doesn't provide control for the ship. Having several sheets loose ("to the wind") could cause the ship to rock about drunkenly. Before settling on the standard usage of "three sheets", a scale used to be employed to rate the drunkenness of a person, with "one sheet" meaning slightly inebriated, and "four sheets" meaning unconscious. A better description relates this phrase to a square rigged ship sailing on the wind, on a bowline as they say. With the three windward sheets hauled all the way forward, in or to the wind, the ship will stagger like a drunken sailor as she meets the waves at an angle of 60 degrees to the beam. For loose sheets to have this effect there would have to be six loose sheets, three to windward and three to leeward. Also, unless all the upper sails secured to the yards were also loosed having the course sheets loose would not produce any change in a ship's motion except to reduce its forward speed a bit.
- (idiomatic) Drunk.
- That late in the evening, he was three sheets to the wind and had long since stopped making sense.