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]

The Collaborative International Dictionary of English v.0.48:

Joist \Joist\, v. t. [imp. & p. p. Joisted; p. pr. & vb. n.
   Joisting.]
   To fit or furnish with joists. --Johnson.
   [1913 Webster]

WordNet (r) 3.0 (2006):

joist
    n 1: beam used to support floors or roofs