Archaic KJV Word
Fain
Modern equivalent: gladly (but inadequate)
What Was Lost
The prodigal son's degradation. He would 'fain have filled his belly with pig food' -- he was desperately, humiliatingly willing to eat slop. The word captured the specific emotion of being reduced to gladness for scraps. No modern word combines willingness, desperation, and humiliation in a single syllable.
Closest Survivor in Modern English
none (the word has no living descendant in common English)
Peak Usage (1611)
KJV Luke 15:16 -- 'He would fain have filled his belly with the husks that the swine did eat'
Died ~1700
Simply died with no replacement that carried its specific emotional tone. 'Fain' (from Old English faegen 'glad/willing/eager under constraint') expressed desperate willingness -- glad to do something even though the circumstances were terrible.
What Replaced It
“gladly”
Cheerful; fain was gladness born of desperation, not happiness
“willingly”
Voluntary; fain implied being willing because you had no better option
“eagerly”
Enthusiastic; fain was eagerness born of starvation, not excitement