Our word stupid is over four and a half centuries old! It was borrowed, with the same definition, in 1540 from Middle French stupide, and not long before that, stupide came from the Latin word stupidus. This carried somewhat different connotations: it meant something more along the lines of "amazed". You were confounded, bedazzled, beyond words. Something was so awesome that it made you stupid, or, in this case, stupidus. In verb form, this was stupere, which meant "to be stunned" and is the source of the words stupefy, stupor, and stop. Likely through Proto-Hellenic, this is reconstructed as being a derivative of the Proto-Indo-European root stewp or stupe, meaning "to hit", as in you were "hit" into being stunned. Earlier, it might have been another violent action like "push" or "beat", but you get the idea. Usage of the word stupid has been increasing almost exponentially since the 1970s- probably coinciding with the exponential rate of human reproduction, ha.
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