Archaic KJV Word
Lord
Modern equivalent: sir/master
What Was Lost
The ownership claim. 'Jesus is Lord' was the earliest Christian confession, and it was political dynamite in Rome where 'Caesar is Lord' was mandatory. To declare kyrios Iesous was to declare that Caesar was NOT Lord -- you had one absolute owner and it was not the empire. It was a pledge of total allegiance that could get you killed.
Closest Survivor in Modern English
Lord (still used in worship but heard as 'respectful religious title' rather than 'absolute sovereign owner of my life')
Peak Usage (1611)
KJV Philippians 2:11 -- 'Every tongue should confess that Jesus Christ is Lord'; Deuteronomy 6:4 -- 'The Lord our God is one Lord'
Died still used but emptied (~1800)
Hebrew Adonai ('absolute master/sovereign owner') and Greek kyrios ('supreme authority') were drained by English aristocratic usage. 'Lord' became a parliamentary title rather than a declaration of absolute ownership.
What Replaced It
“sir”
Polite address; Adonai meant 'the one who owns me completely'
“master”
Closest but carries slavery connotations that obscure the willing submission dimension
“boss”
Workplace authority; lord meant the one who holds total claim on your life, death, and loyalty