dog-eat-dog

Example: “Wall Street can be a dog-eat-dog world.”

Origin: A deliberate inversion of a classical proverb, and the reversal is the whole point. The Latin saying canis caninam non est — dog does not eat dog — was well known and meant that even the lowest and most brutal creatures observe some limit, refusing to prey on their own kind. English writers turned it inside out to describe a world in which that last restraint has failed, so the phrase does not merely assert that competition is vicious but makes a specific claim: things have deteriorated past the point where even the standard traditionally credited to animals still holds. That is why it carries more moral weight than an ordinary description of rivalry, and why it is nearly always used as an indictment rather than a neutral observation.