To make something fantastic or to create fantastic variations or elaborations on something; to give something a fantastical character.
From fantastic plus the verb-forming suffix -ate meaning 'to make or become.' This is a rare, constructed verb that doesn't appear in most dictionaries, created for literary or specialized effect.
Words like 'fantasticate' are what linguists call 'nonce formations'—words invented for a single occasion that follow perfectly logical English rules, showing that native speakers can create new words on the fly without sounding wrong.
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