νεφρός G3510
nephrós · nef-ros' · Noun Masculine
of uncertain affinity; a kidney (plural), i.e. (figuratively) the inmost mind:--reins.
1 Occurrence in the King James Bible
Revelation 2:23And I will kill her children with death; and all the churches shall know that I am he which searcheth the reins and hearts: and I will give unto every one of you according to your works.