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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]