Eureka (word)
"Eureka" comes from the Ancient Greek word "heúrēka", meaning "I found (it)", which is the first person singular perfect indicative active of the verb "heuriskō" "I find". It is closely related to heuristic, which refers to experience-based techniques for problem solving, learning, and discovery.
Papuk
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.