Bad-mouth v. 诽谤，说坏话
Divorced parents shouldn't bad-mouth each other in front of the kids.
leave a bad taste in sb's mouth
The way he was talking to the old man left a bad taste in my mouth.
Big mouth 多嘴的人
God you've got a big mouth! Don't you ever stop talking about other people's business?
They’ve got a lot of mouthwatering desserts at the party last night.
By word of mouth
A: "How did you hear about our bakery?”
B: "Oh, by word of mouth. My dad told me about it.”