from The American Heritage® Dictionary of the English Language, 4th Edition
- adj. Persistently calm, whether when facing difficulties or experiencing success; not easily upset or excited.
from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.
- adj. not easily perturbed or excited or upset; marked by extreme calm and composure
un- + flap (“upset, stir, scandal, controversy”) + -able (Wiktionary)