Where does “arьmò” come from?

arьmò (Proto-Slavic) comes from Proto-Indo-European h₂er- — to fit, to fix, to put together.

Ancestry of “arьmò”, step by step

Step	Language	Word	Meaning
1	Proto-Indo-European	h₂er-	to fit, to fix, to put together

arьmò — Etymology, Origin & Word History | Etymology Explorer