Tim Bowen sheds some light on the origins and definition of the phrase three sheets to the wind.
The adjective ‘drunk’ is said to be one of the English words with the most synonyms. Certainly there are a large number of expressions that describe various degrees of intoxication, not all of them polite. The expression three sheets to the wind is a fairly mild way of saying that someone is very drunk.
Like many other expressions in English, it has its origins in seafaring. In the days of sailing ships, some ships had three sails per mast. At the end of each sail there was a cord called a sheet that could be tightened or loosened depending on the strength of the wind. In windy conditions it was the custom to loosen the sheets so that the sails would fill out and make full use of the conditions. The ship would then pitch and roll in the stormy sea. Drunks staggering about were said to resemble the movements of a ship with three sheets to the wind and the expression soon began to be applied to them too.