Sinew

Sinew (pronounced sin-you)

(1) In anatomy, the classical name for a tendon.

(2) Figuratively, that which gives strength or in which strength consists; a supporting factor or member; mainstay the source or an expression of strength, power or vigor (usually as in the plural).

(3) In literature, an alternative name for muscle.

(4) A string or chord, as of a musical instrument (now rare except poetically).

Pre 900: From the Middle English sinu, from the Old English seonowe, an oblique form of the nominative sionu (sinew) from the Proto-Germanic senawo.  It’s the root also of the Old Saxon sinewa. the Old Norse sina, the Old Frisian sine, the Middle Dutch senuwe, the Dutch zenuw, the Old High German senawa and the German sehne.  Ultimate root was the primitive Indo-European sai- (to tie, bind), source also of the Sanskrit snavah (sinew) and syati & sinati (to bind), the Avestan snavar and the Irish sin (chain); related was the Hittite ishai- & ishi- (to bind).  By the late fourteenth century, sinewy meant “made of sinews, and by 1570 the sense had extended to "tough and stringy.  Sinew is a noun and verb and sinewy, sinewed & sinewous are adjectives; the noun plural is sinews.

Lindsay Lohan Boxer by solidwheel02 on Deviantart, a little artistic licence taken with the sinews.  Still, either way, she looks good.