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.
- rule of engagement
- rule of law
- rule of the road
- rule of three
- rule of thumb
- rule-proving
- ruler
- ruler cult
- rulered
- ruleress
- rule-right
- rulering
- ruler joint
- rulership
- ruler-straight
- Rules
- Rules Committee
- ruleship
- rules-maker
- rule something out (or in)
- rule-staff
- rule the roost
- rule with a rod of iron
- ruling
- ruling class