Up until the fifteenth century, the word bad wasn't really being used at all. It seems to have emerged from an obscure English dialect to become popular in vulgar speech and eventually replace evil as the default antonym for good. Because of the word's early rarity and apparent lack of cognates, the origin is officially unknown, but the most widely accepted theory is that it comes from Old English bæddel, meaning "hermaphrodite". The loss of the -el would match other spelling changes, and the ash is just an old way of writing the "a" sound in "bad". Semantically, it could have just been adopted as a generic curse word that got less severe over time. Bæddel does have relatives in other languages but also has an uncertain etymology, likely deriving from an earlier Proto-Germanic source.
Hello! I'm Adam Aleksic, a sophomore studying linguistics and government at Harvard University, where I founded the Harvard Undergraduate Linguistics Society. I also have disturbing interests in politics, vexillology, geography, board games, conlanging, and law.