Jeremiah 6:30 - Ellicott's Commentary On The Whole Bible

Reprobate silver. — Better, as in the margin, refuse silver; the dross and not the metal; so worthless that even Jehovah, as the great refiner, rejects it utterly, as yielding nothing. The adjective and the verb have in the Hebrew the emphasis of being formed from the same root, Refuse silver... because Jehovals had refused them.