Old Church Slavonic

Old Church Slavonic
Old′ Church′ Slavon′ic
(or Slav′ic) n.
peo the oldest attested Slavic language, extant in a group of manuscripts written before c1100, largely scripture and liturgical translations that were the outcome of Cyril and Methodius's mission to the Moravian Slavs in 863
Abbr.: OCS 2)
Etymology: 1875–80

From formal English to slang. 2014.