'Sugar' Shane Mosley -- Gets Last FAT ASS Laugh in BKB War

Accusing former champ "Sugar" Shane Mosley of being "fat as s**t" ... came back to bite BKB Middleweight champ Gabe Rosado in the ass -- dude did NOT make weight for his title bout!

Mosley wasted NO TIME ripping Rosado a new one by posting a pic of Curtis Stevens -- the fighter he trained for Saturday night's fight with Rosado. He captioned it, "The face you make when your bitch ass opponent doesn't make weight. #fatass #BitchAss #F**kRosado"

Shane's celebration was brief. BKB says Rosado had 60 minutes to cut weight, and he managed to lose 2 pounds in that time! His final weigh-in for Saturday night's PPV bout was 159.

Of course, 24 hours ago ... Rosado was the one fat shaming Shane, but shoe's on the other bloated foot now.