Source : Webster's Revised Unabridged Dictionary (1913)

Looker \Look"er\, n.
   One who looks.

   {Looker-on}, a spectator; one that looks on, but has no
      agency or part in an affair.

            Did not this fatal war affront thy coast, Yet
            sattest thou an idle looker-on ?      --Fairfax.

Source : WordNet®

looker-on
     n : someone who looks on [syn: {onlooker}]