Where does “kaplja” come from?

kaplja (Proto-Slavic) comes from Proto-Slavic -ja, from Proto-Indo-European -yeh₂.

Ancestry of “kaplja”, step by step

StepLanguageWordMeaning
1Proto-Slavic-jaSuffix creating feminine action nouns, usually...
2Proto-Indo-European-yeh₂—
Every word from Proto-Indo-European -yeh₂ →Every word from Proto-Slavic -ja →