2. Happening by chance; belonging but not essential; unplanned, unintentional, unpremeditated, uncalculated; unexpected, fortuitous; chance, random: "The meeting of Dale and Mike on the street was purely accidental."
"Accidentally" is often mispronounced and misspelled.
The correct 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.
James accidentally tallied the incidental car repair costs inaccurately and scared Jane silly.
Later, James tried to explain the accidental nature of his calculations to Jane which greatly relieved her.