"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).