I don't remember how it came about, but I suggested once to Pippi that a good alternative to swearing (especially at another person) was to simply use a normal word in another language. And I gave her the example "Fenster" - which is simply 'window' in German. Nothing offensive there.
Fenster came up in conversation today (yes, I'll admit it seems to be one of the few German vocabulary words that I can remember consistently) (and yes, we do have odd dinner time conversations.) Not sure what led to this but Harry wanted me to spell a German word for him. I spelled Fenster.
Pippi shouted, "Window" and reminded me of my hint to use that as a 'curse' if necessary.
Then she turned to her brother, pointed at him and said "Fenster! I can see right through you!"