E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
means true to the plumbline, but is generally used to express that self-possession which arises from perfect self-confidence. We also talk of a dancers aplomb, meaning that he is a perfect master of his art. (French. à plomb.)
Here exists the best stock in the world men of aplomb and reserve, of great range and many moods, of strong instincts, yet apt for culture.Emerson: English Traits, p. 130.