Before the days of sonar, ships would determine the depth of the water by posting a sailor at the front of the ship with a lead weight attached to a long rope.
The rope would have a knot in every fathom (6 feet).
The sailor's job was to swing the weight forward into the water and, when the ship passed directly over the weight, determine the depth of the water.
As retrieving the lead weight was hard work, a lazy sailor would swing the lead as if he were about to release it.
He would keep swinging it until such time as he thought he was being watched and only then release it into the water.
Therefore, a sailor who was swinging the lead was being lazy.