天高地厚

Explanation: The majority of the original description of heaven and earth, the latter described the End very deep. Also be used to do the difficult, serious and significant relationship. 　　Usage: Joint type; as object; metaphor to the difficult, serious 　　Source: "Book of Songs Xiaoya first month": "that the days of cover high, did not dare not Board; that land to build thick, and dare not ridge." 　　Examples: Protege by the mentor, good friend, San Francisco, today serve little, only as little as A examination only, ~, did not pay in case. (Ming Feng Meng "Jingshitongyan" Volume XVIII)