The Greek word for fantasy is φαντασία (fantasia). It means the ability of the mind to recall in mind, a situation as they are again happening or make things in mind. It is sure very close with verb φαίνομαι (fenome – from this word originates also the English word phenomenon) which means appear / look. So because the mind makes things look real it became the word fantasy. Based on Plato it wasn’t the mind but the soul that made “phantoms based on reality”. Phantoms also originate from the Greek word φαίνομαι.
The meaning of fantastic wasn’t used with a meaning of something extraordinary good in ancient Greek but in modern Greek, the meaning is valid as it came from the English use of the word. In Ancient Greece fantastic was only something that was a creation of imagination
Etymology of word fantasy (fantastical, fantastic)
