The Collaborative International Dictionary of English v.0.48:

Lookout \Look"out`\, n.
   1. A careful looking or watching for any object or event.
      [1913 Webster]

   2. The place from which such observation is made.
      [1913 Webster]

   3. A person engaged in watching; a sentinel; a sentry.
      [1913 Webster]

   4. Object or duty of forethought and care; responsibility.
      [Colloq.]
      [1913 Webster]

   on the lookout for in search of; looking for.
      [PJC]