Where does “Poljakinja” come from?

Poljakinja (Serbo-Croatian) comes from Serbo-Croatian -inja, from Proto-Slavic -yni, from Proto-Slavic -ni — one.

Ancestry of “Poljakinja”, step by step

StepLanguageWordMeaning
1Serbo-Croatian-injaSuffix appended to words to create a feminine...
2Proto-Slavic-yniDenominal, forming feminine noun forms....
3Proto-Slavic-nione
Every word from Proto-Slavic -ni →Every word from Proto-Slavic -yni →Every word from Serbo-Croatian -inja →