Floyd had begged the court to let him finish out his sentence for domestic violence on house arrest ... because he's dehydrated and malnourished and it's taking a toll on his body and putting his boxing career in jeopardy.

But a judge DENIED Mayweather's request -- ordering him to serve out the remainder of his 3-month sentence behind bars.

According to court docs obtained by TMZ, the judge called B.S. on Mayweather's claim that he's dehydrated behind bars -- saying the boxer's condition is "self-induced as water is made available to [Floyd] twenty-four hours a day."

As for Floyd's gripe that he's only consuming a fraction of the calories he needs -- the judge says it's because "[Floyd] chooses not to eat the food provided."

The judge also balked at Floyd's complaint that he can't train at a world class level while serving his time -- saying, "While the training areas and times provided to Floyd may not be consistent with his prior regimen, he is indeed provided sufficient space and time for physical activity if he so chooses."