My favourite is idiot and idiomatic, both coming from the word ἴδιος - literally meaning one's own. While idiot usually implies someone lacking intelligence, an ῐ̓δῐώτης (idiotes) is a person who lacks perspective, namely, one that is not their own.

In other words, an idiot is someone so caught up in their own perspective they are incapable of engaging in fruitful public conversation. Don't know what Plato would think of Twitter then . . .