The math in aftermath has nothing to do with "math" as we know it today. In fact, in Middle and Old English, it referred to the mowing of a lawn, and the aftermath would be the clippings that remained after you cut your grass, usually with a scythe. Over time, this morphed into a more extreme type of consequence, but still retains the farming definition in agricultural jargon. After- was the same in Proto-Germanic and comes from Proto-Indo-European hepo, meaning "off". Math, meanwhile, Came from Old English mæð and PIE me, which still meant "to cut grass". The other type of math, the one we learn in school, is an abbreviation of mathematics, comes from Greek, and is entirely unrelated. Aftermath entered the English language since 1496 and has been increasing in usage since then.
Adam Aleksic, a rising sophomore studying linguistics and government at Harvard University, has been described as the internet's sixth most famous etymologist. He also has disturbing interests in politics, vexillology, geography, board games, conlanging, and law.