TIL the original word for a Bear was ‘Arkto’ but our proto-Germanic ancestors started referring to them as ‘bear’ which meant ‘brown one.’ This practice came from a superstitious fear that speaking the predators name would summon him. This is one of the earliest examples of a euphemism.
We know what the word they were avoiding was- Arkto. We can construct it from other languages.
Half of all spoken language is descendant from Proto-Indo-European, a language spoken by a Eurasian Steppe culture that migrated out from their homeland somewhere in either Russia or the Caucasus (this is why Caucasian means “White European descent”). They spread so far and so wide that fingerprints of their language are found in almost all of Europe (looking at you, Suoma and Magyars, ya weird bastards), and much of West Asia and the Indian subcontinent. In the 1890s, linguists were beginning to notice a weird quirk of Latin, Greek, and Sanskrit: They all sounded alike. Two of which developed independently and one of which was all the way out in fucking India, so what the hell? Well, linguists already knew that languages had history at that they can diverge, so a few started trying to identify cognates- words in these languages that might be related. As this genealogy of language developed, more tongues were added to the family tree, and soon enough, most of Europe is on the list, along with the Turkic languages and Armenian (which sounds like no other language out there so this was a big deal).
In the same way we pieced together these cognate languages, we know what the word was- because it is still used in other languages. See, it was the Germanic branch of the PIE family that used a new name.
The German word for “Bear” is Bär /bɛɐ/. The Nordic word is almost always Bjørn /bjœːɳ/, with little variation among them. The Dutch word is Beer and the Frisian (English’s close cousin) is Bear.
Yet the Latin word is Ursus. The Italian word is Orso. Spanish word is Oso. The Russian word is Medvedv. The Polish word is Niedźwiedź. The Turkish word is Ayi. The Uzbek word is Ayiq. And so on.
From living and dead langauges, we can tell the fluid change of the word, until we can arrive at the original. The Slavic languages are all going by a Proto-Indo-European word meaning “Honey-Eater”, though it doesn’t appear the word for “bear” was taboo to the Slavic branch. The Latin languages are all descendant from Old Italic “Orssos”, which is congate with the Proto-Greek word “Arkos” (Fun fact: Arctic. The Northern One).
All these mean the original word for Bear in that language was something like *h₂ŕ̥tḱos.