from
The Collaborative International Dictionary of English v.0.48
Joist \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.
[1913 Webster]