A steep, sudden drop downward (like a plane diving nose-first), or a sudden collapse or decline.
Compound of 'nose' + 'dive,' originally from aviation in the early 1900s. Extended to describe any sudden sharp decline.
Pilots use controlled nosedives to escape enemy fire in combat, and ironically, a well-executed nosedive requires incredible skill—the term evolved to mean 'sudden disaster' even though pilots planned them carefully.
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