Archaic KJV Word
Corn
Modern equivalent: grain
What Was Lost
The universality of the word. In the KJV, corn meant grain -- whatever grain your land grew. Ruth gleaned barley, not maize. Jesus's 'corn of wheat' was a single grain kernel, not an ear of American corn. The word connected every reader to their own local bread grain. American English broke that connection by assigning corn to one specific crop that did not exist in the biblical world.
Closest Survivor in Modern English
kernel (from the same Germanic root, 'a corn/grain' diminutive -- a little corn)
Peak Usage (1611)
KJV Ruth 2:2 -- 'Let me now go to the field, and glean ears of corn'; John 12:24 -- 'Except a corn of wheat fall into the ground and die'
Died ~1800 (in American English; British English still uses 'corn' for grain generally)
American English captured the word for a single New World plant (maize), while British English kept the original broad meaning. The split created a transatlantic reading error for every grain passage in the KJV.
What Replaced It
“grain”
Generic and industrial; corn carried the warmth of a staple crop, whatever the local grain was
“wheat”
Specific species; corn was the default word for whatever grain sustained your community
“maize”
The opposite problem -- a single species from the Americas replacing the universal term