According to TMZ, Amber Rose wants Wiz Khalifa's mom to back the hell off ... so she's lawyering up to get a restraining order against her ex-mother-in-law.

Sources close to Amber tell TMZ ... Amber's furious that Katie Wimbush-Polk's hatred for her has spilled into the courtroom. Katie has sued Amber for defamation, claiming Amber's allegation Katie's an unfit grandmother is a big fat lie.

We're told Amber believes it's just a revenge lawsuit because Katie can't let go of the fact Amber left her son 3 years ago, and Katie has tried to make Amber's life a living hell ever since the split.

Amber wants a restraining order to keep her and Wiz's 4-year-old son, Sebastian, far away from Katie ... fearing that exposing Sebastian to Katie's hatred for Amber will hurt the kid.