Without sight or clear information; in a blindfolded manner or state of ignorance.
From 'blindfold' plus the adverb suffix '-ly.' This archaic or rare formation creates an adverb meaning to act in the manner of someone who is blindfolded.
This is a genuinely rare word—so rare that most modern dictionaries might not list it. Its existence shows how productively English lets speakers create new adverbs even from compound verbs, allowing someone to say 'they proceeded blindfoldly' to mean both literally and figuratively.
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