Floyd Mayweather: My Retirement Diet ... Burgers, Shakes, Waffles

Well, Floyd Mayweather's definitely eating like he's not coming back ...

The boxing legend pigged out like a maniac during a stop at Archie's Burgers and Shakes in Manchester, England yesterday ... stuffing himself with unhealthy deliciousness.

There are reports Mayweather is interested in bringing the restaurant chain to the US -- possibly opening one in Las Vegas.

And get this ... they've even got a dessert named after his pal Justin Bieber.

Who's hungry?