Where does “ryssys” come from?

ryssys (Ingrian) comes from Ingrian -s, from Proto-Finnic -nci, from Proto-Uralic -mte.

Ancestry of “ryssys”, step by step

StepLanguageWordMeaning
1Ingrian-sUsed to form the inessive case; in, inside
2Proto-Finnic-nciForms ordinal numbers from the corresponding...
3Proto-Uralic-mte—
Every word from Proto-Uralic -mte →Every word from Proto-Finnic -nci →Every word from Ingrian -s →