Old English seonowe, oblique form of nominative sionu "sinew," from Proto-Germanic *senawo (cognates: Old Saxon sinewa, Old Norse sina, Old Frisian sine, Middle Dutch senuwe, Dutch zenuw, Old High German senawa, German Sehne), from PIE root *sai- "to tie, bind" (cognates: Sanskrit snavah "sinew," Avestan snavar, Irish sin "chain").