What does ‘It’s a dog-eat-dog world’ mean? And a doggy-dog world?

It’s a dog-eat-dog world

…is a bit of a cliché. Where does it come from? The first relevant quotation in the OED is from a 5 August 1794 headline in the Gazette of the United States: ‘Dog eat dog’. The next quotation (1822) is from a British source, and then The Times of 30 December 1854 has an example which explains the meaning very well:

‘It was dog eat dog—tit for tat... the customers cheated us in their fabrics; we cheated the customers with our goods.’