Archaic KJV Word
Reins
Modern equivalent: inmost being
What Was Lost
The ancient embodied psychology. The Hebrews located the deepest, most hidden emotions in the kidneys (kelayot) -- the organs buried deepest in the body, hidden from all eyes. When God searches the reins and hearts, He examines both the hidden emotional core (kidneys) and the will/mind (heart). Two organs, two dimensions of the inner person.
Closest Survivor in Modern English
renal (medical term from the same Latin root, preserving the kidney connection)
Peak Usage (1611)
KJV Psalm 7:9 -- 'The righteous God trieth the hearts and reins'; Revelation 2:23 -- 'I am he which searcheth the reins and hearts'
Died ~1800
The word 'reins' (from Latin renes 'kidneys') -- meaning the kidneys as the seat of deep emotions and secret thoughts -- was overtaken by 'reins' as leather straps for controlling horses, making divine examination of kidneys read as God holding horse equipment.
What Replaced It
“mind”
Intellectual; reins-as-kidneys meant the deepest, most hidden part of a person's emotional life
“inmost being”
Vague; reins specifically located deep emotions in the physical body
“conscience”
Moral judgment; reins encompassed desires, fears, and motivations below conscious awareness