In a clear, definite, and unmistakable manner; without doubt or hesitation.
From Latin decidere meaning 'to cut off, determine,' from de- 'off' + caedere 'to cut.' The English word emerged in the 17th century, with the adverbial form developing to express certainty and definiteness.
This word literally means 'cut off' from doubt - when you're decidedly sure about something, you've metaphorically cut away all uncertainty. It's fascinating how many words about certainty derive from cutting or separating metaphors.
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