The Secret of the Doorknob!

For the past several weeks, Gabe has been playing a game where he will go into a room (usually the bathroom) and close the door.   Then he will wait for one of us to notice that he is missing, and knock on the door to let him out.    So, today he starts to play this game, but finds that the door is already closed!   Frustrated, he starts to muck about with the doorknob, and, Heavens to Betsy!, he figures it out!   Having mastered this most ineffable of adult mysteries, he of course completely forgets the elaborate game of Peek-a-Door that he was going to play.    He spent the next thirty minutes just going into the bathroom and closing the door behind him, then opening the door and coming out, closing the door behind him.    I’m now very much proud of him, and also very much afraid.

What does the phrase “Heavens to Betsy” mean, anyway?