all bark and no bite

A commonly-used idiom. When someone is "all bark and no bite," it means that he or she is either: 1. Threatening to do something to you, but is not really willing to do it. OR 2. Acting all tough, intimidating, and agressive, but is too cowardly and chicken and/or also not strong enough to be willing to even throw one single punch at you. This idiom is so called, because these kinds of people are likened to dogs who keep barking at you to try to make you afraid, but are actually not going to bite you.