Source : Webster's Revised Unabridged Dictionary (1913)

Sulky \Sulk"y\, n.; pl. {Sulkies}. [From {Sulky}, a.; -- so
   called from the owner's desire of riding alone.]
   A light two-wheeled carriage for a single person.

   Note: Sulky is used adjectively in the names of several
         agricultural machines drawn by horses to denote that
         the machine is provided with wheels and a seat for the
         driver; as, sulky plow; sulky harrow; sulky rake, etc.