shén​mì​mò​cè

enigmatic

神秘莫测 (神祕莫測) shén​mì​mò​cè
enigmatic idiom
puzzling and difficult to interpret, especially regarding a person's behavior, expression, or whereabouts
unfathomable idiom
extremely mysterious and impossible to measure or predict, often used for natural phenomena or complex situations

Frequency

Written text
0.642 per million

Sources

Definitions
CC-CEDICT
Frequency data
BCC Corpus (BLCU)