From Proto-Italic *<span class="searchmatch">sei</span>. Cognate with Latin sī. *<span class="searchmatch">sei</span> if se sepis De Vaan, Michiel (2008) Etymological Dictionary of Latin and the other Italic Languages...
*sei-ne Latin: sīn *sei-we Latin: sīve, seu *<span class="searchmatch">sei</span> kʷis <span class="searchmatch">Volscian</span>: sepis (“if someone”) Latin: sī <span class="searchmatch">Volscian</span>: *<span class="searchmatch">sei</span> ^ De Vaan, Michiel (2008) “sī, sīc”, in Etymological...