Strong’s Greek
G3510 — nephros
The entry below is Strong’s own definition (1890, public domain), and every verse where the word occurs.
νεφρός
nephros — pronounced nef-ros'
of uncertain affinity; a kidney (plural), i.e. (figuratively) the inmost mind:--reins.
1 occurrence in the KJV.
Revelation 2:23
And 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.