Colder than flugen

My grandparents in Southern Kentucky used the phrase to describe a bitterly cold day as:

"It's colder than flugen!". I am not sure where it is from or what the spelling is. I have looked for it spelled flugen and floogen but have not found anyone who knew the origin.

It could be derived from the Swedish town Flugen. It looks as if it could turn pretty cold there. Do you have any Swedish connections?