It is just a word. It means to delay or the opposite of "advance" and is still used like that in industries like aviation and terms like fire removedant.

But when you use it to call someone stupid, that's when you're using it in the context of ablism and as a slur.

Similar to the term for a female dog, which is still used in veterinary medicine and research to mean an actual dog that is female. Though like the R word, the context as a slur is gradually discouraging its use even as its original meaning because people don't want to risk having it misunderstood.