unflappable

un- +‎ flap (“to upset, stir”) +‎ -able

• IPA^(key): /ʌnˈflæpəbəl/

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unflappable (comparative more unflappable, superlative most unflappable)

1. Remaining composed and level-headed at all times; impossible to fluster; not becoming frustrated or irritated easily. [from 1954]
   • 1959, The Economist, volume 190:

     The whole thing looked remarkably like an implacable force meeting an unflappable object.

   • 1978, James D. Margach, The abuse of power: the war between Downing Street and the media from Lloyd George to Callaghan:

     It was for this reason that when Lord Hailsham first described Harold Macmillan as "unflappable'", it became an instant catchword, distancing the incoming Prime Minister from his predecessor

• (remaining composed under pressure): collected, cool, even-keeled

• (remaining composed under pressure): flappable

• flappable
• unflappability
• unflappableness
• unflappably