phlegm (n.)

mid-14c., fleumatik, "having the temperament formerly supposed to result from predominance of the bodily humor phlegm" (cool, calm, self-possessed, and in a pejorative sense, cold, dull, apathetic;) late 14c., "composed of phlegm (the bodily humor); containing phlegm," from Old French fleumatique (13c., Modern French flegmatique), from Late Latin phlegmaticus, from Greek phlegmatikos "abounding in phlegm" (see phlegm). Related: Phlegmatical; phlegmatically.

A verry flewmatike man is in the body lustles, heuy and slow. [John of Trevisa, translation (late 14c.) of Bartholomew Glanville's  "De proprietatibus rerum"]

early 15c., fleumi, "caused by an excess of phlegm (the bodily humor);" mid-15c., "sluggish;" from phlegm (q.v.) + -y (2). In reference to mucous or watery discharge by 1540s.

Proto-Indo-European root meaning "to shine, flash, burn," also "shining white" and forming words for bright colors.

It forms all or part of: beluga; Beltane; black; blancmange; blanch; blank; blanket; blaze (n.1) "bright flame, fire;" bleach; bleak; blemish; blench; blende; blend; blind; blindfold; blitzkrieg; blond; blue (adj.1); blush; conflagration; deflagration; effulgence; effulgent; flagrant; flambe; flambeau; flamboyant; flame; flamingo; flammable; Flavian; Flavius; fulgent; fulminate; inflame; inflammable; phlegm; phlegmatic; phlogiston; phlox; purblind; refulgent; riboflavin.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit bhrajate "shines;" Greek phlegein "to burn;" Latin flamma "flame," fulmen "lightning," fulgere "to shine, flash," flagrare "to burn, blaze, glow;" Old Church Slavonic belu "white;" Lithuanian balnas "pale."