SLAB Line, in sea-language, a small cord passing up behind a ship's main-fail, or fore-fail, and being reeved through a block attached to the lower part of the yard, is thence transmitted in two branches to the foot of the fail, to which it is fastened. It is used to truss up the fail as occasion requires, and more particularly for the convenience of the pilot or steerman, that they may look forward beneath it as the ship advances.