Farrah Abraham is on the verge of having a real fight on her hands -- a legal one, anyway -- if she keeps talking smack.
A lawyer repping celeb boxing promoter Damon Feldman has fired off a cease and desist letter to Farrah threatening to sue her ass if she continues trash talking him and the event ... slated to go down Saturday in Atlantic City.
The lawyer, John A. Gallagher of the Gallagher Law Group, calls Farrah's bashing defamatory. In the letter, obtained by TMZ, Gallagher says Feldman kept his end of the deal .