When sailing close to shore it is important to keep an eye on the depth of the water. In the age of sailing ships this was done by dropping a line over the side of the ship with a lead weight fastened to the end. To swing the lead was considered an easy job and eventually became a term meaning one who is either avoiding hard work or taking and easy option. Hence in modern terms if you ‘swing the lead’ you are slacking off.
Thanks Peta, thats a great alternative.