The Collaborative International Dictionary of English v.0.48:

Trestle \Tres"tle\, n. [OF. trestel, tresteay, F. tr['e]teau;
   probably from L. transtillum a little crossbeam, dim. of
   transtrum a crossbeam. Cf. Transom.] [Written also
   tressel.]
   1. A movable frame or support for anything, as scaffolding,
      consisting of three or four legs secured to a top piece,
      and forming a sort of stool or horse, used by carpenters,
      masons, and other workmen; also, a kind of framework of
      strong posts or piles, and crossbeams, for supporting a
      bridge, the track of a railway, or the like.
      [1913 Webster]

   2. The frame of a table.
      [1913 Webster]

   Trestle board, a board used by architects, draughtsmen, and
      the like, for drawing designs upon; -- so called because
      commonly supported by trestles.

   Trestle bridge. See under Bridge, n.
      [1913 Webster]