It's Eckspanner and it's a term for file folder with elastic bands like this one:
I first encountered the word because I shoped some office material for me and some colleagues. Not knowing the right term for this folder I ended up with the wrong ones.
Later the word arouse my interest because another colleague told me that this could also be a term for his dog.
Let's get to the bottom of this. The first part of the word "Eckspanner" is "Eck" which literally means corner.
The second part of the word is "spanner" which either means tightener or more colloquial voyeur.
Since his dog likes to observe people from the corner of the balcony he came up with this idea.
Here is a sketch for a more visual explanation:
