What is the X in X-mas?
early 14c. (late 13c. in Anglo-Latin), from Old French giste "beam supporting a bridge" (Modern French gîte), noun use of fem. past participle of gesir "to lie," from Latin iacere "to lie, rest," related to iacere "to throw" (see jet (v.)). Notion is of wooden beam on which boards "lie down."
ceiling or floor support in building construction. Joists-of timber, steel, or reinforced concrete-are laid in a parallel series across or abutting girders or a bearing wall, to which they are attached, usually by metal supports called joist hangers, or anchors