Where does “ար-” come from?

ար- (Old Armenian) comes from Old Armenian եհար-, from Proto-Indo-European h₁ésh₂r̥ — blood.

Ancestry of “ար-”, step by step

Step	Language	Word	Meaning
1	Old Armenian	եհար-	—
2	Proto-Indo-European	h₁ésh₂r̥	blood

ար- — Etymology, Origin & Word History | Etymology Explorer