The Collaborative International Dictionary of English v.0.48:

Johnny \John"ny\, n.; pl. Johnnies.
   1. A familiar diminutive of John.
      [1913 Webster]

   2. (Zool.) A sculpin. [Local cant]
      [1913 Webster]

   Johny Crapaud, a jocose designation of a Frenchman, or of
      the French people, collectively.
      [1913 Webster]

WordNet (r) 3.0 (2006):

Johnny
    n 1: `Johnny' was applied as a nickname for Confederate soldiers
         by the Federal soldiers in the American Civil War;
         `greyback' derived from their grey Confederate uniforms
         [syn: Rebel, Reb, Johnny Reb, Johnny, greyback]