Altrussisch

German

German Wikipedia has an article on:

altostslawische Sprache

Etymology

alt (“old”) +‎ Russisch (“Russian”)

Proper noun

Altrussisch n (proper noun, language name, genitive Altrussisch or Altrussischs, alternative nominative (used with the definite article) Altrussische, alternative genitive Altrussischen, no plural)

Old Russian, Old East Slavic (the Old East Slavic language)
Synonym: Altostslawisch