Pappig - does this word derive from "Pappe" (=cardboard)?

But where is the connection with cardboard?
Papp or pappe was also used in the past as a word for children's porridge. The verb "pappen" in the sense of feeding someone porridge is also related to it. Nowadays, the verb "pappen" is generally used colloquially for "to eat".

So the word 'pappig' has nothing to do with the cardboard itself. But cardboard is mixed with paste and initially has a porridge-like consistency, hence the connection to porridge.