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:

Trimming \Trim"ming\,
   a. from Trim, v.
   [1913 Webster]

         The Whigs are, essentially, an inefficient, trimming,
         halfway sort of a party.                 --Jeffrey.
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   Trimming joist (Arch.), a joist into which timber trimmers
      are framed; a header. See Header. --Knight.
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