Archaic KJV Word
Rock
Modern equivalent: foundation
What Was Lost
The cliff-fortress. In the Judean wilderness, a tsur was a massive rock formation with caves, overhangs, and high ground -- a natural fortress where David literally hid from Saul. 'The Lord is my rock' was not 'God is stable' but 'God is the massive cliff I hide behind when the army is hunting me.'
Closest Survivor in Modern English
rock (still used but heard as 'stable thing' rather than 'cliff-fortress you run to for survival')
Peak Usage (1611)
KJV Psalm 18:2 -- 'The Lord is my rock, and my fortress'; 1 Corinthians 10:4 -- 'That Rock was Christ'
Died still used but metaphor-ized (~1800)
Hebrew tsur/sela ('crag/cliff-fortress/massive immovable geological formation that provides shelter and vantage') was flattened into a generic metaphor for 'stability.' The military, geographical, and survival dimensions evaporated.
What Replaced It
“foundation”
Structural support; the rock was also a fortress, a hiding place, and a high ground vantage point
“stability”
Abstract quality; the rock was a specific, tangible, massive reality you could hide behind, climb on, and defend from
“strength”
Internal quality; the rock was external -- something you fled to, not something you generated