This is a misspelling; the correct spelling is 'accidentally,' meaning something happened unintentionally or by chance rather than on purpose.
The correct word 'accidentally' comes from 'accident' (Latin 'accidere' meaning 'to happen to') plus '-al' and '-ly' suffixes. The misspelling 'accidently' is extremely common because people mispronounce it in their heads, dropping the second 'c' sound.
This is the most commonly misspelled word in English—even autocorrect gets confused! People accidentally misspell 'accidentally' constantly, which is hilariously fitting given the word's meaning. Linguists call this 'hypercorrection,' and it happens because we naturally drop the middle 'c' when we say the word aloud.
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