mod. not subject to distraction; imperturbable. : She is totally unflappable.
Dictionary of American Slang and Colloquial Expressions by Richard A. Spears.Fourth Edition. Copyright 2007. Published by McGraw Hill.
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Word Origin & History
unflappable
1958, from un- (1) "not" + flap (v.) + -able. Originally used in ref. to Harold Macmillan, British P.M. 1957-63.