Floyd Mayweather has been telling people close to him that he's considering a 51st fight -- and he's been training like a maniac in the gym ... but there has been NO official movement ... yet. 

Here's the deal ... people close to Floyd say he's still in fighting shape and ready to go. He even posted video training in his Vegas gym. 

The obvious matchup ... a Conor McGregor rematch -- especially with the money they made in the first fight.