I'll admit it: I said fleg-um for the first sixteen years of my life. Properly pronounced flem, phlegm is the term for mucus that you cough up. But to understand its etymology, you need to understand humorism. This has nothing to do with comedy; humorism is actually an ancient and inaccurate medical belief conceptualized by Hippocrates which states that there are four fluids in the human body: blood, black bile, yellow bile, and phlegm. Phlegm occurs when you have a cold, so obviously it's caused by too much heat. So this all comes from the Ancient Greek word phlegma, which described that cold, wet humor (this, by the way, through Old French fleume and Latin phlegma). Back to phlegma. Because of the "heat" theory, it simply meant "inflammation" earlier on, and before that, it meant "burn" as phlegein. This is from phlox, which meant "flame", and is ultimately from the Proto-Indo-European root bhleg, or "to shine".
Adam Aleksic, a leading contender for valedictorian of his high school, is a 215-month-old boy with disturbing interests in etymology, vexillology, geography, and law. Adam would like to one day visit Tajikistan and probably isn't spying for the Uzbek government.
The Etymology Nerd