
Addendum (5/23/2012): As was doing research for my book, Wild Things in the German Language, I came across some explanations. According to one of them, Frechdachs is a mnemonic to help students of Latin remember that audax means frech in that language. I find this explanation somewhat more plausible than a second one, which claims that frech survives in Frechdachs in the now obsolete meaning of "brave" and that badgers got a reputation for bravery because they were willing to defend their burrow, when they had young, even against larger predators.
[Source: Wild Things in the German Language: Kindle version | iBook version]