Where does “руб” come from?

руб (Serbo-Croatian) comes from Proto-Slavic rǫbъ, from Proto-Slavic rǫbati — to cut, chop.

Ancestry of “руб”, step by step

Step	Language	Word	Meaning
1	Proto-Slavic	rǫbъ
2	Proto-Slavic	rǫbati	to cut, chop

руб — Etymology, Origin & Word History | Etymology Explorer