Although “on accident” is wrong, it is easy to see why it is so prevalent.
- “On purpose” is grammatically correct, so many people assume that “on accident” is also acceptable (but it isn’t).
- As children, especially, we’ve undoubtedly told parents and teachers on numerous occasions that “It was an accident,” which sounds just like “It was on accident.” This might explain why “on accident” sounds right.
Some people have claimed that “on accident” is used more commonly by Americans who live in the South. But having never lived in the South, we can neither verify nor refute this claim.
I can’t stand people saying ‘on accident’.
I gotta say, from my experience, the American South bit is true. A lot of people say it here, in fact, I never knew it was incorrect. But I’ve noticed you’ll see/hear children using it more than adults, who are more likely to say accidentally. “On accident” sounds decidedly young, to me, sorta like “quit it”.