Source : Webster's Revised Unabridged Dictionary (1913)

Joist \Joist\, n. [OE. giste, OF. giste, F. g[^i]te, fr. gesir
   to lie, F. g['e]sir. See {Gist}.] (Arch.)
   A piece of timber laid horizontally, or nearly so, to which
   the planks of the floor, or the laths or furring strips of a
   ceiling, are nailed; -- called, according to its position or
   use, {binding joist}, {bridging joist}, {ceiling joist},
   {trimming joist}, etc. See Illust. of {Double-framed floor},
   under {Double}, a.