armus.

Inherited from Proto-Italic *ar(a)mos, perhaps from Proto-Indo-European *h₂erH-mo-s (“arm”). The full-grade pre-form is perhaps also attested in Serbo-Croatian раме (rȁme, “arm”). However, other Indo-European cognates, such as Sanskrit ईर्म (īrmá, “arm, forequarter”), reflect a zero-grade form *h₂r̥H-mó-s. Schrijver, however, considers it impossible for the Latin term to reflect such a zero-grade PIE term, as otherwise PIE *HRHC yields Latin *RăC, such as in rapiō (“to snatch”), from *h₁reh₁p-. In contrast, Prósper suggests a derivation from *Hə́rH.mo-, from *h₂r̥H-mó-s, though this theory relies on own proposed sound law *#CṚ́HC > CaR.C. In any case, perhaps ultimately from Proto-Indo-European *h₂er- (“to join, fit, attach”). Latin cognates include arma, armentum, artus, ars; external cognates include Ossetian арм (arm, “hand”), Bulgarian ра́мо (rámo, “shoulder”), and English arm.