Lacking confidence in one's own ability, worth, or fitness; timid or shy. She gave a diffident smile when asked to speak in front of the class.
Reserved or restrained in manner. His diffident demeanor made it difficult for him to make new friends.
Archaic. Lacking or marked by a lack of trust or faith; distrustful. He gave the newcomer a diffident glance, unsure whether to believe his story.