I am reading From Luther's Epistle Sermons. The text is 1 Peter, chapter one, v. 13-16. The familiar text has St. Peter exhorting the Christian reader to prepare for living the life of faith. "Gird up" are the familiar words.
Luther advises his congregation to prepare for hardship in the faith but he also urges his people not to go crazy about it as some have done. He compares such a person to an ass, a common sort of rhetorical device, I suppose, to engage his audience.
The ridiculous ass in his analogy can become "obstinate and, going on the ice, break a bone."
The point is for a person to maintain self-control and internal discipline.
Since we are iced in in Minnesota I thought that this old German folk image was hilarious. I wonder if there are other examples of an obstinate ass breaking a leg bone in German folk literature.