on accident

Adverb

 * 1) 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).