Joist

A long, strong piece of wood, steel, or concrete that supports floors or ceilings in a building. Many joists are placed side by side to hold up the structure.

From Old French “joiste,” meaning “beam or support,” related to “jouster” (to join or meet), from Latin “iungere,” “to join.” The word came into English building vocabulary in the Middle Ages. The idea is of beams that join and support the structure.