Archaic KJV Word
Trust
Modern equivalent: believe in
What Was Lost
The body weight. Batach was a physical image -- you lean your entire body weight onto something and let it hold you. 'Trust in the Lord' was not 'believe God will come through' but 'throw yourself down onto God and let Him bear your full weight.' Lean not on your own understanding -- lean on Him, literally.
Closest Survivor in Modern English
trust (still used but understood as mental confidence rather than bodily leaning)
Peak Usage (1611)
KJV Psalm 37:5 -- 'Trust in the Lord'; Proverbs 3:5 -- 'Trust in the Lord with all thine heart; and lean not unto thine own understanding'
Died still used but weakened (~1900)
Hebrew batach ('to throw yourself down upon/lean your full weight on/be securely supported') weakened to 'believe someone will do what they say.' Trust lost its physical, weight-bearing dimension.
What Replaced It
“rely on”
Calculated dependence; batach was throwing your full weight onto God without reservation
“count on”
Expected reliability; batach was bodily surrender, not mental assessment
“have confidence in”
Self-referential; batach was about the trustworthiness of the object, not the trust of the subject