The Collaborative International Dictionary of English v.0.48:
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. [1913 Webster]WordNet (r) 3.0 (2006):
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: watchdog, guard dog]