The idiom of the week for this first week in November is Bad Mouth. I have found that the teenager population; here where I am currently that is, use this idiom rather a lot.
These definitions come from The Free Dictionary. Many thanks to them.
bad-mouth someone or something:
to say negative things about someone or something.
“Mr. Smith was always bad-mouthing Mrs. Smith. They just didn’t get along. John bad-mouths his car constantly because it is too small for him.”
to say unpleasant things about someone or something, especially in order to spoil other people’s opinions of them.
“She’s always bad-mouthing her colleagues. Bad-mouthing the police is hardly an original occupation.”
I personally use this idiom rather a lot; as it seems that there are many negative speaking people in this world.