Source : Webster's Revised Unabridged Dictionary (1913)

Grayback \Gray"back`\, n. (Zo["o]l.)
      (a) The California gray whale.
      (b) The redbreasted sandpiper or knot.
      (c) The dowitcher.
      (d) The body louse.

Source : WordNet®

grayback
     n 1: `johnny' was applied as a nickname for Confederate soldiers
          by the Federal soldiers in the American Civil War;
          `grayback' derived from their gray Confederate uniforms
          [syn: {Rebel}, {Reb}, {Johnny Reb}, {Johnny}]
     2: a dowitcher with a gray back [syn: {Limnodromus griseus}]
     3: a sandpiper that breeds in the arctic and winters in the
        southern hemisphere [syn: {knot}, {Calidris canutus}]