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]

Moby Thesaurus II by Grady Ward, 1.0:

45 Moby Thesaurus words for "on the lookout for":
   after, agape, agog, all agog, anticipant, anticipating,
   anticipative, anticipatory, awaiting, certain, confident, delving,
   digging, eager, expectant, expecting, fishing, forearmed,
   forestalling, forewarned, gaping, hopeful, in anticipation,
   in expectation, in search of, loaded for bear, looking for,
   looking forward to, nosy, not surprised, optimistic, out for,
   out for bear, poking, prepared, probing, prying, ready, sanguine,
   searching, sure, unsurprised, waiting, waiting for, watching for