Source : Webster's Revised Unabridged Dictionary (1913)

Trailer \Trail"er\, n.
   A car coupled to, and drawn by, a motor car in front of it;
   -- used esp. of such cars on street railroads. Called also
   {trail car}.

Trailer \Trail"er\, n.
   One who, or that which, trails.

Source : WordNet®

trailer
     n 1: someone who takes more time than necessary; someone who lags
          behind [syn: {dawdler}, {drone}, {laggard}, {lagger}]
     2: an advertisement consisting of short scenes from a motion
        picture that will appear in the near future [syn: {preview},
         {prevue}]
     3: a large transport conveyance designed to be pulled by a
        truck or tractor
     4: a wheeled vehicle that can be pulled by a car or truck and
        is equipped for occupancy [syn: {house trailer}]