诡秘莫测

Explanation: Secretive: the mysterious unknown; unpredictable: can not fathom. Mystery of a person's behavior and attitude is difficult to speculate. 　　Usage: Joint type; as predicate, object; containing derogatory 　　Source: Liu Chong Song "after the village for the continued Dibao Juan Shiyi and one real" poem: "Gui from a transverse tongue, Zhongnan secretive track." Thesaurus:  Haunted impermanence, appear and disappear mysteriously, Marvelous and unpredictable