The first time I ever came across Meadow Mari шнуй šnuj ‘holy’, I was quite struck by the word. It resembles nothing in Chuvash, Tatar or Russian, but the initial consonant cluster means that it cannot be a native Mari word.
While used in the modern literary language, the word is notably missing from the dialectal dictionaries compiled by late 19th-century or early 20th-century fieldworkers. As far as I can tell, the first dictionary to include the word is V. M. Vasil’ev’s Марий Мутэр (Moscow, 1926), where it is glossed ‘святой’ with the example sentence
Шнуй пийал дэнэ йумо ий мучашы̆м шуйа маны̆т. The word is not present in the 1956 Mari–Russian dictionary, possibly on account of the anti-religious edicts of the time, but it is found in the 1991 Mari–Russian dictionary and the 10-volume Словарь марийского языка, where it seems to have long become an established feature of the literary language.
An initial cluster is also found in Eastern Mari spaj ‘beautiful, graceful’, a loan from Tatar zipa, in turn from Persian zibā (see Paasonen’s Ost-Tscheremissisches Wörterbuch pp. 112–113), so I looked for some Turkic or Perso-Arabic word of a similar shape for šnuj, but I eventually gave up. As the word has generally been overlooked in discussions of Mari etymology, there the matter rested for some time.
However, I recently saw that Raija Bartens took a look at šnuj in her paper “Marilaista Isä meidän–rukouksen käännöksistä”, a contribution to the Festschrift for Seppo Suhonen Oekeeta asijoo (Helsinki: Suomalais-Ugrilainen Seura, 1998). In coming across šnuj in her survey of Mari translations of the Lord’s Prayer through the ages, Bartens was just as baffled by the word was I was. Noting that it is used only in Christian contexts, Bartens asks if it isn’t simply a corruption of Russian священный.
Oddly, in his История марийского литературного языка (p. 137), I. G. Ivanov lists this word among terminology from the Mari pagan religion that fell out of active use during the rise of the Mari literary language. Not only was this word not used for Mari paganism, but it is the rise of the literary language in the 20th century that propelled this word from some extremely obscure origin to common use by Meadow Mari-speaking writers everywhere.