unflappable
1. [ adjective ] not easily perturbed or excited or upset; marked by extreme calm and composure
Synonyms: imperturbable
Examples:

"hitherto imperturbable, he now showed signs of alarm" "an imperturbable self-possession" "unflappable in a crisis"

Related terms: composed
Similar spelling:   unpalatable
  unplayable
  unflawed
  unappealable
  unpalatability
  unpalatably
  unflavored
  unflattering
  unifoliate
  unflavoured