In a way that can be questioned or doubted; arguably or uncertainly.
Formed from 'doubtable' plus the adverbial suffix '-ly' (from Old English 'lich,' meaning 'like' or 'in the manner of'). This follows standard patterns for creating adverbs from adjectives.
This word is so rare that most spell-checkers don't recognize it, yet it perfectly fills a gap—when you want to say something is done in a manner that invites doubt, like 'He doubtably succeeded' (succeeded, but we can question how much).
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