RobH_en_tn/jer/18/12.md

But they will say

The word "they" refers to the inhabitants of Judah and Jerusalem.

This is no use

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)

what his evil, stubborn heart desires

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)