Old English seonowe, oblique form of nominative sionu "sinew," from Proto-Germanic *sinwō (source also of 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" (source also of Sanskrit snavah "sinew," syati, sinati "to bind;" Avestan snavar, Irish sin "chain;" Hittite ishai-/ishi- "to bind").
adjective suffix, "full of or characterized by," from Old English -ig, from Proto-Germanic *-iga- (source also of Dutch, Danish, German -ig, Gothic -egs), from PIE -(i)ko-, adjectival suffix, cognate with elements in Greek -ikos, Latin -icus (see -ic). Originally added to nouns in Old English; used from 13c. with verbs, and by 15c. even with other adjectives (for example crispy).
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Definitions of sinewy from WordNet
(of meat) full of sinews; especially impossible to chew;