Expression: At sixes and sevens

To be at sixes and sevens is to be confused or badly organized.
Where does this expression come from? I don't know, but I like it! Here are a few examples:

The whole office was at sixes and sevens before Mary came.
We're at sixes and sevens this morning; could we call you back in a few hours?
She always seemed to be at sixes and sevens and her agenda was out of control!