noun
reticence
RET-ih-suns/ˈretɪsns/
Reluctance to speak or act openly.
Her reticence makes it hard to know what she thinks.

Etymology
The core means being silent, with re- perhaps strengthening that meaning. Latin's keeping silent becomes a more specific reserve in English: avoiding saying too much or revealing one's thoughts too freely.
In context
Thenceforward he was so cautious in his replies that Mr. Hilton felt convinced there was some purpose in his reticence.
More examples
- He shows unusual reticence when we ask about his childhood.
- Their reticence about endorsing the plan worries its supporters.
- The interviewer's gentle questions help overcome her initial reticence.
Forms
reticent adjective — Reluctant to speak or act openly.
Similar words
- reserve
- Suggests habitual restraint rather than reluctance in one situation.
- reluctance
- Applies to unwillingness generally, not specifically to being open.