This quote is a metaphorical way of saying that idleness, represented by the sloth, can be more damaging than hard work, represented by labor. The comparison to rust is apt because rust slowly, but surely, eats away at the metal. Similarly, sloth or laziness eats away at a person’s potential and productivity. On the other hand, labor or hard work, even though it might wear a person down physically, can keep a person sharp, much like how a used key stays bright because of constant use.

In terms of personal development, this quote suggests that it’s better to be worn out from hard work than to let your skills and talents rust away from non-use. It emphasizes the value of diligence and perseverance. Even if the work is tough and tiring, it can polish you and make you shine brighter, just like the used key.

In today’s fast-paced world, this idea is more relevant than ever. In a society that values productivity and innovation, being idle or complacent can lead to stagnation. It can cause one to fall behind in their personal and professional growth. On the other hand, those who are constantly learning, improving, and working hard tend to stand out and succeed. This quote could serve as a reminder to keep striving, keep working, and keep using our ‘key’ so it stays bright.