phlegmyoudaoicibaDictYouDict[phlegm 词源字典]
phlegm: [14] Greek phlégma denoted ‘bodily fluid produced by inflammation’ (it was a derivative of phlégein ‘burn’, which went back to the same Indo-European base as produced English flagrant, flame, fulminate, and phlox [18] – in Greek literally ‘flame’). As Latin phlegma it came to be used for ‘body fluid’ in general, and was incorporated into the medieval system of bodily humours as a term for the ‘cold moist humour’, which induced sluggishness (whence the meaning of the derivative phlegmatic [16]).

This came to be associated in the late Middle Ages with ‘mucus, particularly as produced in the respiratory passage’. English acquired the word via Old French fleume as fleume, and did not revert to the latinate form until the 16th century.
=> flagrant, flame, fulminate, phlox[phlegm etymology, phlegm origin, 英语词源]
phlegm (n.)youdaoicibaDictYouDict
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.