Done by accident or without meaning to, not on purpose.
Formed from un- (not) + intentional (from Latin 'intentio' meaning attention or purpose) + -ly (adverb suffix). The word structure itself tells you it means 'not with intention.'
This word is a great example of how English uses negative prefixes to flip meaning—we could say 'accidentally' instead, but 'unintentionally' specifically emphasizes that there was NO intention behind it, which is slightly different from just being an accident.
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