Old English gyldan "to gild, to cover with a thin layer of gold," from Proto-Germanic *gulthianan (cf. Old Norse gylla "to gild," Old High German ubergulden "to cover with gold"), from *gulthan "gold" (see gold). Related: Gilded; gilding. Figuratively from 1590s.
Add unnecessary adornment or supposed improvement. For example, Offering three different desserts after that elaborate meal would be gilding the lily. This expression is a condensation of Shakespeare's metaphor in King John (4:2): “To gild refined gold, to paint the lily ... is wasteful and ridiculous excess.”
[ c. 1800