The definition of aplomb is a strong belief in yourself and the ability to remain calm under pressure.
To know with certainty that you are the best person for the job and then to perform it well is an example of performing a job with aplomb.
self-possession; assurance; poise
Origin of aplombFr, literally , perpendicularity, equilibrium ; from à, to + plomb, the metal lead: see plumb
Self-confident assurance; poise: “It is native personality &ellipsis; that endows a man to stand before presidents or generals &ellipsis; with aplomb” (Walt Whitman).
Origin of aplombFrench, from Old French a plomb, perpendicularly : a, according to (from Latin ad-; see ad–) + plomb, lead weight (from Latin plumbum, lead).