Nonstandard, accidentally; unintentionally; by chance: When Jane was in school, she wrote the word accidently on the black board, and her teacher corrected her with the standardized form with "accidentally".

This word is often mispronounced and misspelled The word has five syllables ac-ci-den-tal-ly.

The use of accidently is considered a gross error and displays a lack of knowledge of what is supposed to be the correct spelling and pronunciation.