Archaic KJV Word
Ghost
Modern equivalent: spirit
What Was Lost
The unity of divine Spirit and human spirit in one word. 'Holy Ghost' and 'gave up the ghost' used the same word because the breath of God and the breath of life were the same gast. When Jesus 'yielded up the ghost,' He released His spirit/breath -- the same kind of breath God breathed into Adam. The Holy Ghost was not a spooky specter but the living breath of God. Ghost stories killed this connection.
Closest Survivor in Modern English
German Geist ('spirit/mind' -- the cognate that retained the full range)
Peak Usage (1611)
KJV Matthew 28:19 -- 'Baptizing them in the name of the Father, and of the Son, and of the Holy Ghost'; Matthew 27:50 -- 'Jesus yielded up the ghost'
Died ~1900
Ghost (Old English gast 'spirit/breath/soul') was colonized by horror and superstition -- Halloween ghosts, ghost stories, hauntings -- until the theological meaning of 'spirit/breath of life' became inaudible beneath rattling chains.
What Replaced It
“spirit”
Latin import that displaced the native English word; spirit is more abstract, lacking the breath/life-force immediacy of ghost
“Holy Spirit”
Correct but severs the connection to 'gave up the ghost' (died) -- the same word for God's Spirit and human life-breath