Without sight; also means without seeing or understanding; following instructions or beliefs without question or thinking.
From 'blind' (Old English 'blind,' related to Old Norse 'blindr') plus the adverbial suffix '-ly,' which turns adjectives into adverbs. The metaphorical meaning (without understanding) evolved alongside the literal one.
The metaphorical use of 'blindly' is powerful in English because we use sight as our metaphor for understanding—we 'see' a point, we 'look at' evidence, and we 'blind' ourselves to inconvenient facts. This shows how much human cognition is actually rooted in visual metaphors.
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