Possible meanings for this idiom are 1) the people do not care about what Jeremiah will say to them. Alternate translation: "We do not care what you say" or 2) The people do not think that anything good will come of what Jeremiah tells them to do. Alternate translation: "What you are saying will not work" (See: [[rc://en/ta/man/translate/figs-idiom]])
Here the word "heart" represents the person's mind or will. Alternate translation: "what his evil, stubborn mind desires" or "whatever evil things that we stubbornly want to do" (See: [[rc://en/ta/man/translate/figs-metonymy]])