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]