unflappable

adj

1. Remaining composed and level-headed at all times; impossible to fluster; not becoming frustrated or irritated easily.

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

   “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”

   “Whereas his predecessor was impetuous, Obama was praised for being contemplative and unflappable. Bush had a gut; Obama had a brain.”

   “Hill’s unflappable performance capped an extraordinary two weeks of public testimony, in which 12 witnesses testified during seven public hearings.”

   “It took a lot to get Wheeler's pulse going, but Lillian's dangerous joke had done the job. The otherwise unflappable woman was flushed with adrenaline.”