Gild the lily is an idiom

Idiom

Idiom is an expression, word, or phrase that has a figurative meaning that is comprehended in regard to a common use of that expression that is separate from the literal meaning or definition of the words of which it is made...

 that refers to wastefully and unnecessarily adding to something that is already perfect. The term is a derived from a misquotation from William Shakespeare's

William Shakespeare

William Shakespeare was an English poet and playwright, widely regarded as the greatest writer in the English language and the world's pre-eminent dramatist. He is often called England's national poet and the "Bard of Avon"...

 play The Life and Death of King John: "To gild refined gold

Gold

Gold is a chemical element with the symbol Au and an atomic number of 79. Gold is a dense, soft, shiny, malleable and ductile metal. Pure gold has a bright yellow color and luster traditionally considered attractive, which it maintains without oxidizing in air or water. Chemically, gold is a...

, to paint the lily, to throw a perfume on the violet

Violet (plant)

Viola is a genus of flowering plants in the violet family Violaceae, with around 400–500 species distributed around the world. Most species are found in the temperate Northern Hemisphere; however, viola species are also found in widely divergent areas such as Hawaii, Australasia, and the Andes in...

, 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."