The quality or state of being headlong; the characteristic of acting rashly or in a precipitous manner.
From 'headlong' plus the noun-forming suffix '-ness' (Old English '-nes'). This demonstrates the productive nature of English suffixation, creating abstract nouns from adjectives.
English allows us to convert almost any adjective into a noun by adding '-ness'—'headlongness' is grammatically perfect but sounds archaic because we now prefer saying 'recklessness' instead.
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