This is a tough one
Spelling бугръ means that word is not from modern Russian because such spelling was used before the Russian language reform of 1918 -
Code:
https://ru.wikipedia.org/wiki/Реформа_русской_орфографии_1918_года
бугръ theoretically could be Old Slavonic of бугор which means a hillock or a hump
If you could give me a context I might be able to give you a better explanation