Jeremiah 6:30

Reprobate silver shall men call them, because the LORD hath rejected them.

The verse pronounces the Lord's rejection and judgment upon the impenitent people of Judah. That rejection fell because they persisted in rebellion and corruption. It fell for their wickedness and for forsaking Him.
To be called reprobate silver is to be called refuse silver, counted as worthless and refused. The name fits because they had built their spiritual lives with the dross of apostasy and unbelief.
Even when thus rejected they were urged to repent. Repentance meant to build anew with lasting and acceptable materials. Those materials are faith, obedience, and good works.