When do we say ‘gild the lily’?

William Shakespeare used a similar expression in his play King John: ‘To gild refined gold, to paint the lily… is wasteful and ridiculous excess’. Over the years, the phrase got shortened to just ‘gild the lily’. Gild means to cover with a thin layer of gold. Shakespeare referred to the lily because it’s a pretty flower, and covering it with gold to make it more beautiful would be unnecessary. So, the phrase means to spoil something which is already beautiful by adding something extra.