What does Boxing Day have to do with boxing?
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.
Thick, sticky, stringy mucus secreted by the mucous membrane of the respiratory tract, as during a cold or other respiratory infection.
One of the four humors of ancient and medieval physiology, thought to cause sluggishness, apathy, and evenness of temper.