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 freshman studying linguistics at Harvard University, has been described as the internet's sixth most famous etymologist. He has disturbing interests in words, vexillology, geography, board games, limericks, and law, and he loves writing about himself in the third person.
The Etymology Nerd