This appears to be a misspelling or variant of 'fascinatingly'; if valid, it would mean 'in a fascinating manner' with an unusual spelling.
If this exists as a variant, it would derive from a non-standard form of fascinate, possibly from confusion with 'fascinate' vs. 'fascintate' in historical texts.
Sometimes dictionaries capture the ghost of old spelling variations—'fascintatingly' might be a historical variant showing how unstable spelling was before standardization, or simply a documented error that linguists track.
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