On Accident-Purpose???

There seems to be some confusion in my house about what an accident is. Of course this all began with the eldest child who suddenly realized that he could claim that anything he did was an "accident" in an effort to try to get out of being punished. For example....

Easton hitting his sister repeatedly and definitely on purpose with the light saber.
Me: Easton you are losing your Star Wars Wii privileges for the day for hitting your sister with the light saber.
Easton: Oh, that was just an accident.
Me: Really Easton??? Because I'm pretty sure that I saw you hitting your sister and noone was there forcing you to do it.
Easton: OK, well maybe it was just on accident-purpose.

Seriously?!?!?! What exactly does "that was just on accident-purpose" mean? Clearly this is some grey area that has been discovered by my four year-old that is somewhere in between it being on purpose and being on accident. Maybe the boy is a genius. I am considering trying this one out on Michael and seeing how it works for me.

Michael: So I was looking at the account today and saw you've already spent $300 at Target this month.
Me: Oh honey, that was just on "accident-purpose"!!!

Hmmm....wonder how that will go over.