1911 Encyclopædia Britannica/Joist
JOIST, in building, one of a row or tier of beams set edgewise from one wall or partition to another and carrying the flooring boards on the upper edge and the laths of the ceiling on the lower. In double flooring there are three series of joists, binding, bridging, and ceiling joists. The binding joists are the real support of the floor, running from wall to wall, and carrying the bridging joists above and the ceiling joists below (see Carpentry), The Mid. Eng. form of the word was giste or gyste, and was adapted from O. Fr. giste, modern gîte, a beam supporting the platform of a gun. By origin the word meant that on which anything lies or rests (gésir, to lie; Lat. jacere).
The English word “gist,” in such phrases as “the gist of the matter,” the main or central point in an argument, is a doublet of joist. According to Skeat, the origin of this meaning is an O. Fr. proverbial expression, Je sçay bien où gist le lièvre, I know well where the hare lies, i.e. I know the real point of the matter.