Source : Webster's Revised Unabridged Dictionary (1913)

One-horse \One"-horse`\, a.
   1. Drawn by one horse; having but a single horse; as, a
      one-horse carriage.

   2. Second-rate; inferior; small. [Slang, U.S.]

Source : WordNet®

one-horse
     adj : small and remote and insignificant; "a jerkwater college";
           "passed a series of poky little one-horse towns" [syn:
           {jerkwater}, {pokey}, {poky}]