Uncertain.
Per Vasmer, perhaps from Proto-Indo-European *meh₂d-, and cognate with Ancient Greek μύσος (músos, “pollution, desecration”), Ancient Greek μυσαρός (musarós, “vile, covered with shame”), Old Irish mosach (“unclean”), Low German mussig (“dirty”).
Per Preobrazhensky and Trubachyov, cognate with dialectal terms бу́сор (búsor, “junk, old stuff”), бу́сырь (búsyrʹ), бу́сорь (búsorʹ), and probably borrowed from a Turkic language with alternation between /b/ and /m/; Trubachyov in particular characterizes Vasmer's derivation as unconvincing.
An expressive connection with сор (sor, “rubbish, trash”) is also suggested.
му́сор • (músor) m inan (genitive му́сора, uncountable, relational adjective му́сорный, diminutive мусоро́к)
From the pre-1917 acronym МУС (MUS) for Моско́вский уголо́вный сыск (Moskóvskij ugolóvnyj sysk, “Moscow Criminal Detective Department”).
му́сор • (músor) m anim (genitive му́сора, nominative plural мусора́, genitive plural мусоро́в, diminutive мусоро́к)
Borrowed from Russian му́сор (músor).
му́сор • (músor) m pers (genitive му́сора, nominative plural мусори́, genitive plural мусорі́в)
| singular | plural | |
|---|---|---|
| nominative | му́сор músor |
мусори́ musorý |
| genitive | му́сора músora |
мусорі́в musorív |
| dative | му́сорові, му́сору músorovi, músoru |
мусора́м musorám |
| accusative | му́сора músora |
мусорі́в musorív |
| instrumental | му́сором músorom |
мусора́ми musorámy |
| locative | му́сорові, му́сорі músorovi, músori |
мусора́х musoráx |
| vocative | му́соре músore |
мусори́ musorý |