Originally, an imbecile was a category used by psychiatrists to describe people with an IQ between 26 and 50, higher than an idiot but lower than a moron. This was a legitimate description used in courts to prove insanity, but it eventually grew to be pejorative, along with those other terms. The word, like most medical terminology, was taken from Latin, in this case from imbecillus, which meant "weak". That's composed of the prefix in-, meaning "not" (the n changes to an m before a b or p due to place assimilation) and the root baculum, meaning "stick". Nobody is really sure how to explain that connection; it might involve a convoluted link between being "unsupported" and not having a walking stick. Baculum comes from Proto-Indo-European bak, also meaning "stick".
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.