An Etymological Dictionary of the German Language/seit
seit, prep. and conj., ‘since,’ from MidHG. sît, prep. and conj., ‘since,’ adv., ‘since then,’ OHG. sîd, adv., ‘since then, later,’ conj. ‘since, as, because,’ prep. ‘since.’ Comp. OSax. sîð (also sîðor), ‘later, afterwards, since then, if’; orig. a compar. adv.; comp. Goth. þanaseiþs, ‘further,’ allied to seiþus, ‘late.’ As new equiv. compars. comp. also OSax. sîð-or, OHG. sī̆dôr, MidHG. sī̆der. MidHG. sint, a variant of sît, is implied by sintemal; E. since is based on MidE. sithens, sithen, AS. sîððân.