idios - Ancient Greek word for someone who is ignorant of civil or public affairs, unable to effectively participate or engage in the public sphere, thus, not a citizen as they would require full participation and/or at least the ability to formulate arguments about issues of concern. An ancient Greek citizen would despise someone like this as worse than useless..........
I'm well aware of the problems of Ancient Greek conceptions of who should be able (or was able to) participate in society and that should be recognized as a problem in the origins of this word.
However, for modern audiences, it seems to be useful for describing people who formulate opinions solely on what they feel/believe without any attempt to engage/learn with/from others experiences/perspectives/cultures/beliefs/research
idios kosmos - referred to one isolated in their own world (rather than being a part of a shared world that engaged with others' perceived realities) -- what we would consider solipsism
We should consider the political origins of the word idiot as something that reflects a person's willingness/ability to engage with their society, as opposed to its later nightmarish appropriation as a designation/classification of the genetically unfit (see eugenics).
PS: feel free to suggest other words that might be better.