To tarnish means to dull or discolor the surface of metal, or to damage someone's reputation or the quality of something good.

When silver spoons or copper pennies sit unused for months, they develop a dark, cloudy coating that dulls their shine. That's tarnish, a chemical reaction between the metal and air or moisture. You can polish tarnished silver to restore its gleam, scrubbing away that grimy layer to reveal the brightness underneath.

The word takes on deeper meaning when applied to reputation or achievement. A scientist's groundbreaking discovery might be tarnished by revelations that she faked some of her data. A championship team's victory gets tarnished if people learn they broke the rules. When someone lies to a friend, it tarnishes the trust between them.

What makes tarnish interesting is that it happens gradually, without anyone noticing at first. Metal doesn't turn black overnight, and reputations don't collapse instantly. Small acts of dishonesty or carelessness accumulate like that dull coating on silver. Like polishing silver, you can often restore a tarnished reputation through consistent honesty and hard work, though it takes more effort than keeping it bright in the first place.