Description of several days jankers here.

I really doubt the German origin of the word. I'm German, served my time, and never heard it.
In our big national dictionaries (Grimm's German Dictionary, Duden big dictionary of the German Language) I couldn't find the slightest hint. In older dictionary there are no entries.
The verb janken describes the production of whining, howling, squeaking or squealing sounds.
The substantive Janker is a short Jacket.

Wilfried