'Don't drink too much tonight, you were three sheets to the wind last weekend.'This expression is used to describe someone who is drunk to the point of being unable to stand up straight. The ‘sheets’ here refer to the sails of a windmill rather than bed linen. Windmill operators used to add or remove the number of sails according to the strength of the wind.
One basic rule that they had to follow was to always keep an even number of sails – either two or four – opposite each other in order to keep the windmill balanced and steady. If they ever had three sheets, the windmill became unstable and extremely wobbly, swaying from side to side very much like someone who has enjoyed a little too much alcohol!