“nothing trustworthy in their mouth.” Evil people are very good liars and deceivers for a number of reasons, and so it is important when determining what kind of person someone is to look at the fruit of their lives. Jesus taught us this, and said we will know people by their fruit (Matt. 7:16-20). Sadly, many people are good liars and smooth talkers and they convince naïve people of their evil ideas in spite of the evil fruit their lives bear.
The Hebrew Masoretic text reads “his mouth,” but that could be a corruption because the Septuagint reads the plural and there is some Hebrew manuscript evidence for the plural; and furthermore, the plural reading makes sense.