Etymology: Middle English giste, joiste, from Anglo-French giste, from Vulgar Latin *jacitum, from Latin jac?re to lie
One of parallel timbers stretched from wall to wall, to support the floor-boards. Laid flat earlier, from the later 17th century laid on edge.
(Wood, Margaret. The English Medieval House, 412)
*term definitions retrieved from Netserf’s Medieval Glossary (http://www.netserf.org/Glossary)