The Collaborative International Dictionary of English v.0.48:

Looker \Look"er\, n.
   1. One who looks.
      [1913 Webster]

   2. A person who is physically very attractive, especially a
      beautiful woman.
      [PJC]

   Looker-on, a spectator; an onlooker; one that looks on, but
      has no agency or part in an affair.
      [1913 Webster]

            Did not this fatal war affront thy coast,
            Yet sattest thou an idle looker-on ?  --Fairfax.
      [1913 Webster]

WordNet (r) 3.0 (2006):

looker-on
    n 1: someone who looks on [syn: onlooker, looker-on]