The Collaborative International Dictionary of English v.0.48:

Sinewy \Sin"ew*y\, a.
   1. Pertaining to, consisting of, or resembling, a sinew or
      sinews.
      [1913 Webster]

            The sinewy thread my brain lets fall. --Donne.
      [1913 Webster]

   2. Well braced with, or as if with, sinews; nervous;
      vigorous; strong; firm; tough; as, the sinewy Ajax.
      [1913 Webster]

            A man whose words . . . were so close and sinewy.
                                                  --Hare.
      [1913 Webster]

WordNet (r) 3.0 (2006):

sinewy
    adj 1: (of meat) full of sinews; especially impossible to chew
           [syn: fibrous, sinewy, stringy, unchewable]
    2: consisting of tendons or resembling a tendon [syn:
       tendinous, sinewy]
    3: (of a person) possessing physical strength and weight; rugged
       and powerful; "a hefty athlete"; "a muscular boxer";
       "powerful arms" [syn: brawny, hefty, muscular,
       powerful, sinewy]

Moby Thesaurus II by Grady Ward, 1.0:

69 Moby Thesaurus words for "sinewy":
   able-bodied, acid, athletic, biting, brawny, broad-shouldered,
   burly, cartilaginous, chewy, cohesive, coriaceous, corrosive,
   cutting, driving, durable, effective, fibrous, forceful, forcible,
   gristly, gutsy, hard, hardy, imperative, impressive, incisive,
   lasting, leatherlike, leathery, mighty, mordant, muscle-bound,
   muscular, nervous, penetrating, piercing, poignant, powerful,
   punchy, resistant, robust, ropy, sensational, sinewed, slashing,
   stiff, stout, strapping, striking, stringy, strong, stubborn,
   sturdy, telling, tenacious, thewy, thickset, tough,
   tough as leather, trenchant, untiring, vigorous, viscid, vital,
   well-built, well-knit, well-set, well-set-up, wiry