In a way that happens by chance or without being planned or intended.
From 'accident' (Old French, from Latin 'accidere' meaning 'to happen to' or 'to befall,' from 'ad' + 'cadere' to fall) plus the adverbial suffix '-ally' or '-ily.' The word traces back to the concept of something that 'falls upon' you unexpectedly or unintentionally.
Many major scientific discoveries were 'accidents'—penicillin, Post-it notes, and even X-rays were found by chance! This is why the saying 'fortune favors the prepared mind' matters: accidentally finding something amazing only happens to people observant enough to notice it.
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