taciturn

This term describes a personality trait or a habitual state of silence rather than a temporary mood. It carries a connotation of sternness or a deliberate withholding of information, often suggesting a temperament that is naturally aloof or socially guarded. While similar to being shy, it differs by implying a lack of desire to speak rather than a fear of doing so. It is frequently used in formal or literary contexts to characterize a person who is perceived as cold, mysterious, or intensely private.