"Too often" means "too many times", and "once" means "one time".
So "once too often" means "one time too many".
Suppose that a friend tells you a joke. It's a good joke, so you laugh. Then, a week later, he tells you the same joke again. You laugh again, but not so much. Then, another week later, he tells you that very same joke yet again. This time you don't laugh. You've heard the joke "once too often".