*s(w)ēd, the ablative singular ofProto-Indo-European*s(w)é(“self”) lengthened under phrasal stress. Note there's no hard evidence for a /w/ in Italic, which is likely taken from the possessive pronoun in other branches. The original meaning was "per se, by itself", whence "however, but" as conjunction and "without, away" as preposition, parallel toEnglishonly(“but”).
Doublet ofsē as well assed (q.v.), where the vowel shortenedproclitically (or never lengthened). Cf. the semantically closevē-, which might also be a doublet with loss of /s/. Further related tosuus(“one's own”).
De Vaan, Michiel (2008) “sē; sē-, se-, sō-, so-”, inEtymological Dictionary of Latin and the other Italic Languages (Leiden Indo-European Etymological Dictionary Series; 7), Leiden, Boston: Brill,→ISBN,page549
Julius Pokorny (1959),Indogermanisches etymologisches Wörterbuch, in 3 vols, Bern, München: Francke Verlag