Where does “правосточнославянский” come from?
правосточнославянский (Russian) comes from Russian восточнославя́нский, from Russian славя́нский, from Proto-Slavic slověnьskъ, from Proto-Slavic slověninъ, from Proto-Slavic slovo, from Proto-Balto-Slavic ślawas, from Proto-Indo-European ḱléwos, from Proto-Indo-European ḱlew- — to cover; to incline.
правосточнославянский (Russian): Old East Slavic
Ancestry of “правосточнославянский”, step by step