I've never seen a frog's ass, but it sure doesn't sound like an inviting place to be. Nor does the phrase translate in English to anything that makes any real sense, but to the Hungarian, under a frog's ass is exactly the way Hungarians feel when they are 'down in the dumps' or 'gloomy'.

Written by Scott Savoie
I think one of the reasons why the world economic crisis hasn’t been felt so strongly in Hungary is that there is a general underlying feeling of pessimism here. Even the translation for 'gloomy' here is 'under a frog's ass'.
Whereas in North America we are taught to be positive no matter how bleak the situation, as in
“Everything is great! My dog died, I lost my job, my wife left me... But hey! The Prozac is kicking in and its all good now!”
Hungarians, on the other hand, seem to almost downplay good forturne.
“Oh it is terrible. I just got a big promotion at work...”
History has not been kind to Hungary in general and Budapest specifically. Perhaps the Hungarians have earned a right to be pessimistic. History, and especially recent history, has been tragic for the Magyar nation.
The people’s pessimissim is useful in that when bad times do come, people are more prepared and perhaps a bit vindicated at having been right: Things did turn out badly.
Of course, the downside of this pessimism is that even when things are going well in Budapest, it is not always noticed.
The upside of this gloomy outlook is that when things are going badly in Hungary, it’s kind of like business as usual.
Hunglish.org