During the Tang Dynasty, a scholar named Cui Jiao once lived at his aunt's home while studying. His aunt had a maid who was beautiful, clever, and blessed with a naturally lovely singing voice. Cui Jiao grew deeply fond of her, and the maid secretly returned his feelings, though his aunt remained unaware. Later, due to financial hardship, the aunt sold the maid to a high-ranking official named Yu. The lovers were cruelly separated, and Cui Jiao pined for her, but the official's mansion was impenetrable, and the maid could not leave—they had no chance to meet.
During the Qingming Festival, the scholar Cui Jiao unexpectedly encountered his former maidservant, the woman he had longed for day and night. She stood beneath a tree—though thinner, she appeared even more beautiful. But she now belonged to another household. Cui Jiao dared not approach or call out to her; they could only lock eyes in silent gaze. Overwhelmed with sorrow, he composed a poem to express his feelings: "Princes and nobles chase after her dust, Green Pearl's tears soak her silk scarf. Once inside a marquis's gate, it's deep as the sea; from now on, this heartbroken lover is a stranger to her."
"In ancient times, 'Xiao Lang' generally referred to a man beloved by a woman, describing lovers who become strangers after parting, known as 'Xiao Lang on the Road.' The idiom 'Hou Men Si Hai' describes a noble's mansion as deep as the sea, with strict gates forbidding outsiders, also used to depict lovers or close friends kept apart by social status."
Source: Fan Shu (Tang Dynasty), *Yunxi Youyi*, "Xiangyang Jie"
Meaning of the Idiom: Later, the Chinese idiom "侯门似海" came to describe a noble's mansion as deep as the sea, with strict gates forbidding outsiders, also used to depict lovers or close friends kept apart by social status.