diffident
1. [ adjective ] showing modest reserve
Examples:

"she was diffident when offering a comment on the professor's lecture"

Related terms: reserved
2. [ adjective ] lacking self-confidence
Synonyms: shy unsure timid
Examples:

"stood in the doorway diffident and abashed" "problems that call for bold not timid responses" "a very unsure young man"

Related terms: unconfident
Similar spelling:   diffidently
  diffidence
  dividing
  dividend