What is the origin of the idiom "dog eat dog" and when to use it?

The idiom "dog eat dog" has its origins in the world of dogfighting, where dogs were pitted against each other in fights to the death. Today, the phrase is used more broadly to describe any situation where people are competing fiercely for limited resources or opportunities, and are willing to do whatever it takes to come out on top, regardless of the impact on others.