What was the original word for “bear”?

What the OP may (or may not) be getting at is the fact that lots of Indo-European languages use descriptive words for bear, rather than the original Indo-European. This makes linguists surmise that there was a taboo about naming bears, so that euphemisms took the word’s place. Thus, the Germanic words for bear ultimately mean “brown”, and the Slavic words ultimately mean “honey-eater”.

As discussed in e.g. The Brown One, The Honey Eater, The Shaggy Coat, The Destroyer , the Indo-European word appears to have survived in Greek, Sanskrit, Latin and Celtic, and can be reconstructed as *h₂ŕ̥tḱos. The article suggests that  *h₂ŕ̥tḱos itself may have been a euphemism, meaning “destroyer”.