The quality of not paying careful attention to something; lack of concern or thoroughness.
From 'careless' (care + -less suffix meaning 'without') plus '-ness' forming abstract nouns. 'Care' comes from Old English 'caru' meaning anxiety or concern. The suffix '-less' comes from Old English, making this a purely native English construction.
Carelessness costs economies trillions: NASA's Mars Climate Orbiter crashed in 1999 because one team used metric units while another used imperial—a $327 million lesson in the cost of not paying attention. That's why aircraft have checklists: carelessness kills.
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