sphinx noun [C] (PERSON)

An ancient, imaginary creature with a lion’s body and a human head

A person who is mysterious and whose face and expressions do not show what they are thinking:

• “The actor plays him as a sphinx, with blank eyes and a smile that has no meaning.”

• “On screen she was depicted as a sphynx, an unknowable enigma with no friends or family, and no sense of humour.”

Cambridge Dictionary