APLOMB
aplomb, assuredness, cool, poise, sang-froid
(noun) great coolness and composure under strain; “keep your cool”
Source: WordNet® 3.1
Etymology
Noun
aplomb (usually uncountable, plural aplombs)
self-confidence; poise; composure.
(ballet) The apparent elegance and precision exhibited by a confident, accomplished dancer.
The perpendicular; perpendicularity.
Source: Wiktionary
A`plomb", n. Etym: [F., lit. perpendicularity; plomb lead. See
Plumb.]
Definition: Assurance of manner or of action; self-possession.
Source: Webster’s Unabridged Dictionary 1913 Edition