Phlegmatic

Calm, composed, and not easily excited or upset.

From Middle English fleumatik, flewmatik, flematik, fleumatyke, flewmatyk, from Old French fleumatique, from Latin phlegmaticus, from Ancient Greek φλεγματικός (phlegmatikós), from φλέγμα (phlégma), referring to the humour which ancient Hippocratic and later Galenic medicine associated with mildness, coldness, sluggishness, and indifference. Spelling later altered to resemble the word's Latin and Greek roots, with modern pronunciation following this new spelling.

apathetic, cold-blooded, phlegmish, unflappable, stoic