What is three sheets to the wind origin?

The phrase "three sheets to the wind" refers to someone who is extremely intoxicated and unable to function properly. It is believed that the origin of this expression comes from sailing terminology. In the past, sails on a ship were controlled using ropes or lines called "sheets." If one or more of these sheets were loose or not tied properly, the sail would flap in the wind, causing the ship to sway and lose control. If all three sheets were loose, the ship would be totally out of control, much like a person who is heavily intoxicated. Therefore, the phrase "three sheets to the wind" became a colloquial expression to describe someone who is completely drunk and cannot function. The phrase has been in use since at least the early 19th century.