Aflatoon today is a word that is used for an eccentric, sometimes even a mad person.
As such, the word is a distorted form of Plato, which cannot be spoken in Arabic due to absence of 'P' sound and ended up becoming Aflatoon, and was picked up in Urdu and subsequently by some Hindi speakers, also because of the interesting sound the word makes.
As for the meaning, it probably comes from the questioning, curious nature of Plato, which is considered strange by common people.