sin·ew 
(sĭny)
(sĭny)n.
1. A tendon.
2. Vigorous strength; muscular power.
3. often sinews The source or mainstay of vitality and strength: "Good company and good discourse are the very sinews of virtue" (Izaak Walton).
tr.v. sin·ewed, sin·ew·ing, sin·ews
To strengthen with or as if with sinews.
[Middle English sinewe, from Old English sinewe, oblique form of seonu, sinu.]