In a way that can be officially verified or proven; in a manner that is demonstrably true or authentic.
From 'certifiable' plus the adverbial suffix '-ly' (from Old English 'lice', meaning 'like' or 'in the manner of'), creating an adverb from an adjective.
The phrase 'certifiably insane' became famous in legal and medical contexts because it means someone's mental state is officially documented and verified—not just an opinion, but a legal fact.
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