In a manner that warns or alerts someone in advance.
From 'forewarn' + '-ing' (present participle) + '-ly' (adverb suffix). This creates an adverb meaning 'in a forewarning way,' though it is extremely rare in modern usage.
This word is so obscure you'll almost never encounter it, but it shows how English grammar rules let us theoretically build infinite words—we could say 'he spoke forewarningly' even if nobody really does!
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