Three sheets to the wind meant that one did not have control of the vessel because one had lost control of the sheets or lines. Today the expression is used to refer to someone who is drunk or does not have control of themselves.
While one might assume that the word “sheet” represents the sail of the ship, it actually refers to the line used to control the sail. When several sheets were loose, a ship’s sail would flail wildly about, often causing the ship to appear to be staggering uncontrollably, as if in a drunken state. The expression was used to refer to drunkenness even during the age of sail – when a sailor was just a wee bit tipsy, he was one sheet to the wind. Two sheets described a sailor who was well-oiled, while three sheets to the wind represented a sailor who was a stumbling, slurring mess. Down the hatch!