Also, three sheets in the wind. Drunk, inebriated, as in After six beers he's three sheets to the wind. This expression is generally thought to refer to the sheet—that is, a rope or chain—that holds one or both lower corners of a sail. If the sheet is allowed to go slack in the wind, the sail flaps about and the boat is tossed about much as a drunk staggers. Having three sheets loose would presumably make the situation all the worse. Another explanation holds that with two or four sheets to the wind the boat is balanced, whereas with three it is not. [Mid-1800s]

To be “three sheets to the wind” is to be drunk. The sheet is the line that controls the sails on a ship. If the line is not secured, the sail flops in the wind, and the ship loses headway and control. If all three sails are loose, the ship is out of control.