Coming from the Greek verb φλέγω (phlégō), meaning "I burn, fire, scorch," and the Greek noun φλέγμᾰ (phlégma), meaning "flame, fire, heat," but in a medical context meaning, "inflammation," the word phlegm describes mucus produced by one's respiratory system. Ancient Greeks thought it was an aqueous substance (a humor) caused by the heat of inflammation, hence the name phlegm.