"assurance springing from confidence in oneself," 1828, from French aplomb "self-possession," literally "perpendicularity" (16c.), from phrase à plomb "poised upright, balanced," literally "on the plumb line," from Latin plumbum "(the metal) lead" (see plumb (n.)), of which the weight at the end of the line was made.
The staple figure in novels is the man of aplomb, who sits, among the young aspirants and desperates, quite sure and compact, and, never sharing their affections or debilities, hurls his word like a bullet when occasion requires, knows his way, and carries his points. They may scream or applaud, he is never engaged or heated. Napoleon is the type of this class in modern history ; Byron's heroes in poetry. [Emerson, "Social Aims," 1875]