Quaint or strange, bizarre in form, conception or appearance; unreal.
Dated back to the 15th century, this unused adjective stems from the Middle French fantastique (14th century) and from the Latin phantasticus meaning imaginary. The similar Greek, the much older version, was used to describe individuals with the ability to imagine. The most original meaning comes from the Greek verb phantazien meaning ‘to make visible’.
Why this word?
Fantastical is interesting for so many reasons! Let me already say this- no, it is not a mistake, I am not referring to ‘fantastic’!
Nowadays we use the word fantastic to describe something that is very good, excellence that is out of the ordinary when actually, the ‘out of the ordinary’ part is the only surviving piece of this word’s original meaning. In the original sense, this word used to describe things that can only take place in imagination, distanced from real life. The trivial sense of ‘wonderful’ and ‘marvelous’ was first recorded as late as 1938, but influenced English speakers so much that it is the only sense still being used today.
‘Fantastical’, which originally meant the same a fantastic, already existed when the new sense was coined; I am assuming that the fact that fantastical is simply longer than fantastic- made it less habitual and drove it closer to oblivion.
Now you’re probably asking what the difference between the two is. Although dictionaries show the definition above for both words, looks like in reality it is not at all so. If a child is on his most devilish behavior, we would sound very weird if we were to describe his behavior as fantastic as this adjective is only used for its positive sense.
Speakers are those who define meanings, not dictionaries as it is often thought, and we have redefined it again- fantastic is amazingly good while fantastical remains imaginary.
How to use fantastical?
Simply- in its original sense as describing anything that is unreal, not necessarily in the positive sense of it… When a child severely misbehaves- it is fantastical. When no matter how hard you try, you still can’t understand the reasons to which someone does something- it is fantastical.
When everything is great- it is fantastic.