great self-confidence, composure, or poise.
example: Although the power was out and most of the audience drunk and unruly, she performed her number with admirable aplomb.
“assurance, confidence,” 1828, from French aplomb (16c.), literally “perpendicularity,” 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. (Doug Harper, www.etymonline.com)
Wikipedia: plumb bob