The characters combine literally to mean “difficult than ascending the blue sky,” a phrase that has remained unchanged for over a thousand years due to its powerful imagery.
The soul of this idiom comes from one of China's most revered poets, Li Bai (李白), from the Tang Dynasty (8th century CE). He wrote a famous poem called “The Hard Road to Shu” (《蜀道难》, Shǔ Dào Nán), which described the treacherous, nearly impassable mountain roads leading into the ancient state of Shu (modern-day Sichuan province). The poem's most famous line is: “蜀道之难，难于上青天！” (Shǔ dào zhī nán, nán yú shàng qīng tiān!), which means “The difficulty of the Shu roads is harder than ascending to the blue sky!” Because of this origin, the idiom is not just a phrase; it's a cultural touchstone. It evokes a sense of history, classical literature, and the immense scale of China's geography. Compared to a Western phrase like “it's a Herculean task,” which references Greek mythology, “难于上青天” is rooted in China's own literary and historical tradition. While both signify great difficulty, the Chinese phrase carries a more poetic and slightly melancholic tone, emphasizing the natural, insurmountable obstacle rather than a challenge to be overcome by a hero.
This idiom is alive and well in modern Mandarin. It can be used in a wide range of situations, from serious discussions to lighthearted complaints.
The connotation is almost always focused on the immense difficulty. It's a statement about the obstacle itself, not necessarily a statement of defeat, but it clearly sets the stage for a significant struggle.