DOG EAT DOG -- "The struggle for survival in life or business turns man into an animal." This variation came later than the original "dog doesn't or won't eat dog," according to Gregory Titelman.Dog won't eat dog "means that people of the same type do not destroy one another.'canis caninam non est' is Latin for 'a dog doesn't eat dog's flesh' and 'Cane non mangia cane' is Italian for 'dog doesn't eat dog.' In 1602 Shakespeare used the same idea in 'Troilus and Cressida.' First attested in the United States in 'Modern Chivalry' by Hugh Henry Brackenridge (1748-1816).Dog eat dog is of much later origin." "Random House Dictionary of Popular Proverbs and Sayings" by Gregory Y.Titelman (Random House,New York,1996).

The meaning is : One disreputable person will not harm other disreputable people.