To apply unnecessary ornament – to over embellish.

Shakespeare didn’t coin the term ‘gild the lily’, but he came as close to doing so in King John, 1595:

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.