The quality or state of being fantastic; the degree to which something is extraordinary, imaginative, or unbelievable.
From fantastic plus the noun-forming suffix -ality meaning 'quality or state.' This is a rare, formal noun that elevates 'fantastic' to an abstract concept, often used in philosophical or literary contexts.
The word 'fantasticality' barely appears in modern writing, but when Victorian authors used it, they meant it as the highest compliment—a measure of how much a work transcended mundane reality into something truly imaginative.
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