imperturbable

/ˌim-pər-ˈtər-bə-bəl/

Middle English, from Late Latin imperturbabilis, from Latin in- + perturbare to perturb

adjective

  1. marked by extreme calm, impassivity, and steadiness : serene

cool composed collected unruffled imperturbable nonchalant mean free from agitation or excitement. cool may imply calmness, deliberateness, or dispassionateness. composed implies freedom from agitation as a result of self-discipline or a sedate disposition.