Cincinnati Reds Star -- CHEWED OUT BY FAT ORG. ... After Calling Reporter 'Fat Motherf**ker'

Cincinnati Reds 2nd baseman Brandon Phillips is in deep-fried trouble with a pro-fat person organization after blasting a sports reporter as a "fat motherf**ker" ... TMZ has learned.

The National Association for the Advancement of Fat Acceptance (NAAFA) had their ears (and probably their mouths) open when Phillips went off on C. Trent Rosecrans during a news conference with Reds manager Dusty Baker.

Rosecrans -- who happens to sport a fuller figure -- had gotten on Phillips' bad side for criticizing his on-base percentage ... and while CTR was talking to Baker, Phillips interrupted and called him a "fat f**k" and a "fat motherf**ker."

Baker just sat there ... refusing to call out Brandon.

But the NAAFA is hungry for an apology ... and also probably pizza ... and meatballs ... telling TMZ, "[Brandon's] poor choice of words indicates that he may have missed the fact that verbal aggression and threats are bullying."

The org. says they're especially upset because the Reds participate in anti-bullying campaigns -- "Putting on an anti-bullying face then allowing the blasting of a reporter's body-size is not acceptable."

We've reached out to the Reds -- so far, no word back.