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arma (Noun) "weapon"
12th cent. From Latin arma "weapons." From Proto-Italic *armo- "weapons," earlier probably "tools" or "that which is joined together." Originally an adjective reinterpreted as a noun due to influence from the noun-forming suffix *-mo- (see -mo). From Proto-Indo-European *h2r̥-mo- "fitting." From *h2er- "to join."
Also the origin of the surname Armas.
Aragonese arma, Asturian arma, Portuguese arma, Galician arma, Catalan arma, French arme, Italian arma, Dalmatian jarma, Aromanian armã, Romanian armă, Sardinian àrma
Ancient Greek ἅρμα (árma) "chariot," ἀραρίσκω (ararísko) "to fit," Mycenaean a-ra-ro-wo-a "fitted," a-mo "wheel"
Armenian y-armar "fitting," aṙnem "to make"
Sanskrit r̥tá- "truthful," "oath," ará- "wheel spoke," Old Avestan arə̄m "fitting"

Originally a neuter plural in Latin. In Vulgar Latin, neuter plural words were reanalyzed as feminine singulars (as was the case in Spanish hoja). The change from neuter to feminine must have occurred early in Latin and not after, as the word is feminine even in languages that preserve the neuter case (Asturian, Romanian, etc...).