In a way that is astonishing, stunning, or so beautiful or exciting that it takes your breath away and makes you pause in awe.
From 'breathtaking' (breath + taking, meaning literally to take someone's breath), with the adverbial suffix '-ly' (Old English 'lice'). The phrase became common in 19th-century literary English to describe overwhelming beauty or shock.
Your body actually does hold its breath when startled or amazed—it's an ancient fight-or-flight reflex, which is why this metaphor for awe is neurologically accurate, not just poetic.
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