reticent

Not revealing one's thoughts or feelings readily; reluctant to speak.

• reticence/ˈrɛtɪsəns/(n)沉默
• reticently/ˈrɛtɪsəntli/(adv)沉默地

• reticent about
• reticent to speak
• naturally reticent
• reticent personality

1. 1.She was reticent about discussing her personal life with colleagues. (unwilling to share)
2. 2.He's naturally reticent — don't mistake his silence for rudeness. (quiet by nature)

• reserved/rɪˈzɜːvd/— keeping to oneself
• taciturn/ˈtæsɪtɜːn/— habitually silent
• tight-lipped/ˌtaɪtˈlɪpt/— refusing to speak, informal

from Latin "reticere" = to be silent — re- (intensive) + tacere (to be silent)