What does guilding the lily mean?

To gild something is to apply a coating of gold to it. The expression means to add beauty to something that is already perfect and beautiful. It is excessive and redundant.

In a broader sense, it means to overdo something, to try to add what it already has and doesn't need more of.

The expression comes from Shakespeare and has been incorrectly quoted for more than a century:SALISBURY:

Therefore, to be possess'd with double pomp,

To guard a title that was rich before,

To gild refined gold, to paint the lily,

To throw a perfume on the violet,

To smooth the ice, or add another hue

Unto the rainbow, or with taper-light

To seek the beauteous eye of heaven to garnish,

Is wasteful and ridiculous excess.

King John, 1594

Many of our common expressions are quotations from literature, and many of those are familiar to us as inaccurate quotations.