late 14c., fleem "viscid mucus" (the stuff itself and also regarded as a bodily humor), from Old French fleume (13c., Modern French flegme), from Late Latin phlegma, one of the four humors of the body, from Greek phlegma "humor caused by heat," lit "inflammation, heat," from phlegein "to burn," related to phlox (genitive phlogos) "flame, blaze," from PIE *bhleg- "to shine, flash," from root *bhel- (1) "to shine, flash, burn" (see bleach, v.). Modern form is attested from c.1660. The "cold, moist" humor of the body, in medieval physiology, it was believed to cause apathy.