bad-mouth 
verb [with object] informal
Criticize (someone) behind their back: no one wants to hire an individual who bad-mouths a prior employer...
- As if I had enough to fill my plate, now I have to worry about my ex-best friend bad-mouthing me behind my back.
- We don't have to be character assassins, we don't have to issue screeds, we don't have to be bad-mouthing other people, but we do have to use logic and reason and persuasion and show why our positions really make more sense for America.
- We were willing to let it go, but they are bad-mouthing him.