Three sheets to the wind

In sailing, a sheet is a line (rope, cable or chain) used to control the movable corner(s) (clews) of a sail.

"Sheet" derives from Old English "scéata" meaning the lower corner of a sail.

Fore-and-aft rigs comprise the vast majority of sailing vessels in use today, including effectively all dinghies and yachts. The sheet on a fore-and-aft sail controls the angle of the sail to the wind, and should be adjusted to keep the sail just filled. Most smaller boats use the Bermuda rig, which has two or three sets of sheets:

On the smallest boats, a sheet is often a simple rope, pulled by hand; on larger boats, usually on the mainsheet, it is often a system using several blocks to provide mechanical advantage. At one time, headsail sheets on yachts also involved blocks, but the disadvantages of having large amounts of line and loose blocks free to move all over the forward part of the vessel mean that such systems have been almost universally replaced with single-line sheets holding immense forces but controlled by powerful winches.

Square-rigged vessels are much less common, and are usually large ships. Nevertheless, they too have sheets on the movable corners of their square sails. Unlike fore-and-aft sheets, though, square-rig sheets do not control the angle of the sails (which is performed using braces); instead, they are used to haul the corners of the sails from their stowed positions down towards the tip of the yard below. They are then not adjusted significantly while sailing until the sail is to be handed (put away) again. The lowest sails, the courses, are trimmed using the sheets as these sails are loose footed and are secured to yards only at the head.

The term "three sheets to the wind," meaning "staggering drunk", refers to a ship whose sheets have come loose, causing the sails to flap uncontrolled and the ship to meander at the mercy of the elements.

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Wikipedia