phlegmatic
adjective /fleɡˈmætɪk/
  /fleɡˈmætɪk/
- not easily made angry or upset synonym calm
- a phlegmatic temperament
 
Word OriginMiddle English (in the sense ‘relating to the humour phlegm’): from Old French fleumatique, via Latin from Greek phlegmatikos, from phlegma ‘inflammation’, from phlegein ‘to burn’.