Why is the ninth month called September?
Old English seonowe, oblique form of nominative sionu "sinew," from Proto-Germanic *senawo (cf. 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" (cf. Sanskrit snavah "sinew," Avestan snavar, Irish sin "chain").
sinew sin·ew (sĭn'yōō)
Vigorous strength; muscular power.