3 sheets to the wind

To understand this phrase we need to enter the arcane world of nautical terminology. Sailors' language is, unsurprisingly, all at sea and many supposed derivations have to go by the board. Don't be taken aback to hear that sheets aren't sails, as landlubbers might expect, but ropes (or occasionally, chains). These are fixed to the lower corners of sails, to hold them in place. If three sheets are loose and blowing about in the wind then the sails will flap and the boat will lurch about like a drunken sailor. (credit to phrases.org.uk)