Oslund said people should assume every dog has the potential to bite, including dogs with wagging tails and owners who say, «Oh, he growls at everyone.» (dogids.com)
The Origin: Most dogs do wag their tails when they are happy. (petwave.com)
But a dog wagging his tail with a bias to the left side is showing a negative, anxious response to whatever is happening. (mnn.com)