Agreed, post-mint damage. From the looks of the other side, it has seen considerable circulation. It is unreasonable that a coin with such an obvious defect would have remained in circulation for very long, especially as it would be rejected by vending machines for being too light. Banks are obligated to withdraw and not reissue such badly damaged coins.
For the cause, I would surmise it is either deliberately manufactured, or was perhaps caught in machinery such as a lawnmower or wood chipper.
Don't say "infinitely" when you mean "very"; otherwise, you'll have no word left when you want to talk about something really infinite. - C. S. Lewis