In a manner that is capable of being doubted or questioned; in a way that can legitimately be called into question.
From 'dubitable' (itself from Latin 'dubitabilis') plus the adverbial suffix '-ly.' A rare adverbial form of the already-uncommon adjective 'dubitable.'
This word is so philosophical and rare that it appears mainly in academic papers about epistemology (the study of knowledge)—it's essentially the thought process of questioning things taken to its linguistic extreme.
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