In a fantastic manner; in a way that is extraordinary, imaginative, or amazingly good.
From fantastic plus the adverbial suffix -ly meaning 'in the manner of.' Note this is a non-standard variant; the standard adverb form is 'fantastically' with an extra 'a', but some writers use this variant.
The debate over 'fantasticly' versus 'fantastically' is a real grammar question—English spelling rules conflict here, and both have appeared in published works, showing how language rules are more flexible than we think.
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