"diffident" in the adjective sense
1. diffident
showing modest reserve
"she was diffident when offering a comment on the professor's lecture"
2. diffident, shy, timid, unsure
lacking self-confidence
"stood in the doorway diffident and abashed"
"problems that call for bold not timid responses"
"a very unsure young man"
Source: WordNet® (An amazing lexical database of English)
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