The Collaborative International Dictionary of English v.0.48:
Aplomb \A`plomb"\, n. [F., lit. perpendicularity; ? to + plomb lead. See Plumb.] Assurance of manner or of action; self-possession. [1913 Webster]WordNet (r) 3.0 (2006):
aplomb n 1: great coolness and composure under strain; "keep your cool" [syn: aplomb, assuredness, cool, poise, sang- froid]