Brewer:
Three Sheets in the Wind Unsteady from over-drinking, as a ship when its sheets are in the wind. The
sail of a ship is fastened at one of the bottom corners by a rope called a “tack;” the other corner is left
more or less free as the rope called a “sheet” is disposed; if quite free, the sheet is said to be “in the
wind,” and the sail flaps and flutters without restraint. If all the three sails were so loosened, the ship
would “reel and stagger like a drunken man.”

Tne best explanation of the common phrase I have ever seen. Remember, the "sheet" is not a sail,
it is a rope (a line).