There is a (rare) preposition sēd, sē “without” with the ablative (as in Old Latin sed fraude), and (more commonly) a prefix sē- “without, apart from” (as in securus etc.), and also the conjunction sed “but”. The commonly accepted theory is that these derive from some case form (ablative?) of the reflexive pronoun IE *s(w)e*swe-, as in Latin se, suus. The semantic development would have been something along the lines of “on one’s own” > “without”.