Phlegmatic, rooting from phlegm, is an onomatopoeic term which many associate with a thick, yellow-green bacterial bodily fluid. It was once believed that the phlegmatic temperament was a result of an overabundance of phlegm among the other bodily humours (black bile, yellow bile, blood). Depending on diction, one who is phlegmatic can be described as either an unemotional slob or a composed introvert. The juxtaposition of connotation only proves that there are good and bad things about everyone; it can be either a pejorative or somewhat flattering way of describing a personality.
The term comes from the Ancient Greek word “φλέγμα” (or phlégma), which means fire, heat, or (medical) inflammation, and phlegm. Phlegm, a thick, slimy, and slow-moving bodily substance, is a combination of mucus and bacteria produced by the respiratory system as a response to heat or inflammation in the lungs. Hippocrates (460-377 BC) was the first to link the phlegmatic personality with phlegm. One whose body produces excess amounts of phlegm behaves similarly to the substance. The phlegmatic is generally slow-moving, unemotional, and apathetic in life, expending little energy and doing as little as possible. They are stubborn when it comes to making changes and have a monotonous disposition. To some, the phlegmatic may appear dull, sluggish, and stolid. To others, the phlegmatic may project an aura of calmness, rationality, and stability. They can be easygoing, complacent, and observant, and generally are not the ones to experience mood swings or exaggerate their emotions.
Although the ancient theory of the four temperaments is no longer followed in modern day medicine, the phlegmatic still exists today. One behaves phlegmatically when they have a lot of phlegm—the human body directs energy to the immune system in response to illness, resulting in drowsiness, sluggishness, and inactivity. These conditions are generally frowned upon in modern day society—which leaves the phlegmatic in a difficult position—but can be beneficial in dealing with the fast-pace and stressful aspects of modern day life.
Harper, Douglas. "Online Etymology Dictionary." Last modified 2012. Accessed November 17, 2012. http://www.etymonline.com/index.php?allowed_in_frame=0&search=aggregate&searchmode=none