diffident \DIF-uh-dunt; -dent\ (adjective) - 1 : Lacking self-confidence; distrustful of one's own powers; timid; bashful. 2 : Characterized by modest reserve; unassertive.
"But I am diffident. What's the good of saying I mustn't be diffident when I'm the man who wrote the words and music, when Diffidence is my middle name and my telegraphic address. I can't help being diffident." -- P.G. Wodehouse, 'The Clicking of Cuthbert'
Middle English, from Latin diffīdēns, diffīdent-, present participle of diffīdere, to mistrust : dis-, dis- + fīdere, to trust; see bheidh- in Indo-European roots.