Concise Bible Dictionary:

God’s ancient people in their condition of moral debasement are compared to “reprobate silver,” or “refuse silver” as in the margin (Jer. 6:3030Reprobate silver shall men call them, because the Lord hath rejected them. (Jeremiah 6:30)); or, as is read in Isaiah 1:2222Thy silver is become dross, thy wine mixed with water: (Isaiah 1:22), “thy silver is become dross,” rejected.
In the New Testament the word is ἀδοκιμος, “disapproved,” and is applied to the wicked, and to those also who having engaged in the race fail to reach the goal (Rom. 1:2828And even as they did not like to retain God in their knowledge, God gave them over to a reprobate mind, to do those things which are not convenient; (Romans 1:28); 2 Cor. 13:5-75Examine yourselves, whether ye be in the faith; prove your own selves. Know ye not your own selves, how that Jesus Christ is in you, except ye be reprobates? 6But I trust that ye shall know that we are not reprobates. 7Now I pray to God that ye do no evil; not that we should appear approved, but that ye should do that which is honest, though we be as reprobates. (2 Corinthians 13:5‑7); 2 Tim. 3:88Now as Jannes and Jambres withstood Moses, so do these also resist the truth: men of corrupt minds, reprobate concerning the faith. (2 Timothy 3:8); Titus 1:1616They profess that they know God; but in works they deny him, being abominable, and disobedient, and unto every good work reprobate. (Titus 1:16)). The same word is translated “rejected,” and is descriptive of such as, in spite of gracious ministry, produce only that which is natural to fallen man (Heb. 6:88But that which beareth thorns and briers is rejected, and is nigh unto cursing; whose end is to be burned. (Hebrews 6:8)). It is also translated “castaway” in 1 Corinthians 9:2727But I keep under my body, and bring it into subjection: lest that by any means, when I have preached to others, I myself should be a castaway. (1 Corinthians 9:27), where the apostle Paul represents himself as keeping under his body, and bringing it into subjection, lest having preached to others he himself should be rejected. This appears to indicate the possibility of a man, after having preached the gospel to others, being himself disapproved; failing to reach the goal through lack of self-discipline.
Photo of aluminum dross (left) and aluminum (right).