Where does “si” come from?

si (Old Catalan) comes from Latin sī, from Proto-Italic sei, from Proto-Indo-European só — this; that.

Ancestry of “si”, step by step

StepLanguageWordMeaning
1Latinsīif, supposing that
2Proto-Italicseiso, thus
3Proto-Indo-Europeansóthis; that

Words derived from “si”

Every word from Proto-Indo-European só →Every word from Proto-Italic sei →Every word from Latin sī →