Diffident

diffident [DIFF-uh-dənt]
adj. lacking self-confidence

[Derives from Latin diffīdere, dis- ("absence of")+ fīdere ("trust")]

The fīde in diffident is the same root in words like fidelity and confidence. It means, of course, trust or faith. Thus, to be diffident is to not have faith your own abilities.

Diffident is often used to mean shy ("a wish to escape notice"), timid ("lacking courage"), bashful ("fear of being noticed") or cautious ("aware of the possibility of danger"). I'm sure those uses meet various dictionary definitions. Most who are diffident are also shy, timid, bashful and cautious. However, just because someone is chary ("cautious or careful") or quiet doesn't mean he lacks confidence in himself. My recommendation is to limit diffident to describing someone who lacks personal confidence. Someone who is "characterized by shyness and modesty" is demure. A person who is "insincerely shy" is coy. And the guy who is "ashamed" of himself is abashed.

In Secrets: A Memoir of Vietnam and the Pentagon Papers, Daniel Ellsberg describes a scene where he is doing something which contradicts what he had been taught was right, and so he was not at first confident in his actions, but over time his assurance grows:

For the first half hour I was diffident about discarding secret documents, but before long I was throwing them into the bag with abandon.