The noun diffidence refers to a lack of self-confidence. Your diffidence might be the reason why you never say "hi" to the cute guy or gal in the elevator or why you never ask for a raise.
The noun diffidence comes from the Latin word diffidere meaning "to mistrust" or "to lack confidence." Diffidence is often mistaken for snootiness because people don't understand that the diffident person is shy and lacking in confidence. "They asked him to be the team leader, but he expressed diffidence, saying that he didn't think he had enough time to do the job justice, nor did he think he had enough experience."