Source : Webster's Revised Unabridged Dictionary (1913)

Watchdog \Watch"dog`\ (w[o^]ch"d[o^]g`), n.
   A dog kept to watch and guard premises or property, and to
   give notice of the approach of intruders.

Source : WordNet®

watchdog
     n 1: a guardian or defender against theft or illegal practices or
          waste; "she is the global watchdog for human rights
          abuses"
     2: a dog trained to guard property [syn: {guard dog}]