From Bai Juyi's (
白居易) Tang narrative poem 《
长恨歌》 (Song of Everlasting Sorrow), introducing Yang Guifei:
杨家有女初长成，
养在深闺人未识 — 'The Yang family had a daughter just coming of age, raised in the deep inner chambers, still unknown to the world.'
深闺 is the traditional women's quarters at the back of a household. Today the phrase is almost always applied to under-discovered places, products, or people.
