The realm or practice of fantasy; works of fantasy or the creation of fantastical things; fanciful or imaginative activity.
From fantastic plus the noun-forming suffix -ry meaning 'the practice, product, or domain of.' Similar to how 'archery' is the practice of archery and 'wizardry' is the practice of being a wizard, fantastry is the domain of fantasy.
The word 'fantastry' is almost extinct, replaced by the simpler 'fantasy,' but Victorian writers loved it because -ry words suggest a whole craft or tradition, making 'fantastry' sound like a legitimate art form worthy of serious study.
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