From sharar in the original sense as in shor (compare shorer); a cord, i.e. (by analogy) sinew -- navel.
see HEBREW sharar
see HEBREW shor
see HEBREW shorer
H8306. sharir
[שָׁרִיר] noun [masculine] sinew, muscle (so context suggests); — pluralconstruct בִטְנוֺ בִּשְׁרִירֵי אוֺנוֺ Job 40:16 ("" כֹחוֺמְָתְנָיו).