Definition from Wiktionary, the free dictionary
- Accidentally; not intentionally; because of error, misfortune, or lack of caution.
- I'm sure he backed into the hedge on accident, but the hedge and the car are both a mess now.
Usage notes 
- In the US, on accident is about 1–2% as common, on average, as by accident (COCA). However, usage varies geographically; some regions use on accident much more than others.
- In the UK, on accident is about 3% as common as by accident (BNC).
by accident — see by accident