Our language is always evolving, so much so that dictionaries can't always keep up with us. Such is the case with "fantastic" and "fantastical." Standard dictionaries have yet to catch up as speakers differentiate these two nearly identical words.
For most of us, "fantastic" means "excellent" or "great." However, that's not what it meant historically. When it first came into English from Latin in the 1300s, "fantastic" meant "imaginary," that is, "related to fantasy." Over its history, "fantastic" came to mean l "fanciful," "irrational," and "unrealistic."
It wasn't until the 20th century when "fantastic" took on the more positive "excellent" meaning. It's this meaning that has allowed "fantastic" to rise in popularity, particularly in the second half of the 20th century.
When "fantastical" came into the language in the 15th century, it also meant "imaginary" or "related to fantasy." However, we often see it used in a more negative context, at least in written English. For example, a person who can't face reality may be described has having a fantastical world view.
That's not to say "fantastical " is always negative. If someone tells you their dinner was fantastical, they probably had a really good meal. However, if someone describes a movie as "fantastical," did they really enjoy it, or are they complaining that it was unrealistic? It's not always clear.
It's this ambiguity that has us wondering what's going to happen with the meanings of "fantastic" and "fantastical." Professor Anne Curzan says it's hard to say, but there are a couple of possibilities:
"One [possibility] is that we could maintain or strengthen a distinction, where 'fantastic' mostly means 'excellent,' while 'fantastical' mostly means 'imaginary.' [Or] 'fantastical' could become more and more popular in the 'excellent' meaning. If that were to happen, we might see another word like 'fanciful' pick up steam to mean 'imaginary' or 'unrealistic' if 'fantastical' patterns and mostly means 'excellent.'"
In other words, we'll have to wait and see.
