There are several supposed etymologies of the name Papuk, it's almost certainly not of Croatian origin. One is that it comes by assimilation from earlier Bapuk, where Ba is the name of the Celtic tribe that inhabited the region, and Puk comes from the Indo-European root *peiH, meaning big. However, from the historical sources, it's visible that Papuk was originally a hydronym. Based on that, it's been suggested that the name comes from the repetitive of the Indo-European root *bhogj (to flow), *bhebhogj, so that it means that which flows and flows. However, there are several problems with that etymology. First, it's visible by hydronyms such as Bosut and Bosna that Indo-European *bh gave *b in the local language. Second, *bhebhogj would actually mean flow and flow and not that which flows and flows, so the endings don't match. So, it's been suggested that the name is of Pre-Indo-European origin.