From WordNet (r) 3.0 (2006) [wn]:
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}]
From The Collaborative International Dictionary of English v.0.48 [gcide]:
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]