"cool, calm, self-possessed," and in a more pejorative sense, "cold, dull, apathetic," 1570s, from literal sense "abounding in phlegm (as a bodily humor)" (mid-14c., fleumatik), from Old French fleumatique (13c., Modern French flegmatique), from Late Latin phlegmaticus, from Greek phlegmatikos "abounding in phlegm" (see phlegm).
A verry flewmatike man is in the body lustles, heuy and slow. [John of Trevisa, translation of Bartholomew de Glanville's "De proprietatibus rerum," 1398]
phlegmatic phleg·mat·ic (flěg-māt'ĭk) or phleg·mat·i·cal (-ĭ-kəl)
Of or relating to phlegm.
Having or suggesting a calm, sluggish temperament; unemotional.