Confidence = Competence + Purpose

As engineers, our effectiveness is often measured by non-technical metrics. Objective measures like lines of code can provide misleading results.  You'll likely be assessed by some subjective approximation of competence instead.  Sprezzatura, displaying a sense of ease while getting the job done, is perhaps the best name we have for this.

It would seem intuitive to offer the advice, "appear confident." However, unwarranted confidence can itself be dangerous. There's a well known cognitive bias, the Dunning-Kruger Effect, that shows that confidence is often highest when ability is low.   It's better to coach for competence. When someone lacks competence they should lack confidence.Someone may also appear to lack confidence when they lack purpose. Conceding to someone with a higher purpose is a moral choice. When an ambulance comes speeding along, you yield the right of way. 

Confidence should naturally follow when one has both competence and purpose.  Of these, purpose can be harder to cultivate.  A starting point is for the organization to clearly and regularly communicate vision, purpose, and how each team contributes.  It's up to the manager to further break this down and tie tasks back to motivate individuals.

Sometimes you may not believe in your organization's vision.  Still, it's good to be honest with yourself about what purpose you do believe in and how you can contribute to it in your current situation.  If you can't tie them together, don't rationalize.  Consider what you can do to move in the right direction and act.  If you lock yourself into a situation where you don't believe in the purpose you're working toward, you'll struggle to compete with people that do.  You'll be less effective and miss opportunities.  You'll also probably be unhappy.