AB and a celeb chef named Stefano Tedeschi have been fighting it out in court since July when Tedeschi filed a lawsuit alleging Brown stiffed him out of $38,521.20.

Tedeschi claims he and Brown struck an agreement where AB would pay him to cook for him, his family and his friends for 3 days at the Pro Bowl in Orlando in January 2018.

But, Tedeschi says AB never paid up ... and claims it's all because of a frozen fish head.

Brown eventually called BS on Tedeschi in a motion to dismiss the lawsuit earlier this month ... going scorched-earth on the chef, writing, "[Tedeschi's] complaint is nothing more than a fledgling action to promote the career of a chef who will use any attempt or frivolous claim to seek fame and/or fortune."

Now, Tedeschi is firing back ... in new court docs, Tedeschi is accusing Brown of defamation.

Tedeschi says Brown's words were "slanderous, defamatory and not the type of language one would expect to see in a professional legal document."

Tedeschi -- who wants the court to deny Brown's motion entirely -- added, "This language can only be viewed as an effort to smear [Tedeschi]."

We reached out to Brown's attorney, Darren Heitner, for comment on the new docs ... and he told us, "There was simply no defamatory statement made."