To define phlegmatic, first, think of the word phlegm, they are closely related, which means a mucus-like-liquid produced in the chest, nose, or throat. The Webster's dictionary refers to the word phlegm to define phlegmatic. Phlegm defines as mucus in the lungs or respiratory passages when a person has the flu, chills, or a cold—bad dieting can also cause phlegm and mucus. I guess because phlegm or mucus is blend, cold, and has no real taste or color to associate with, Phlegm is considers phlegmatic.
Phlegmatic defines as someone who is calm, shows no emotions such as excitement, anger, or vitality other than just being. A person who is completely detach from any form of expression like phlegm or mucus: cold, tasteless, or dull. Phlegmatic is like a fortified object, for instance, a sculpture: forever indifferent—remained as it was carved, permanent, unchanged, dull—nothing more, nothing less.