Archaic KJV Word
Oath
Modern equivalent: solemn promise
What Was Lost
The self-curse. An oath was not a strong promise but a conditional self-destruction: 'May God do to me what was done to these animals if I break this oath' -- spoken while walking between the halved carcasses of sacrificed animals. When God swore an oath to Abraham, He invoked Himself as the guarantee, since there was no one greater to swear by.
Closest Survivor in Modern English
oath of office (retains gravity but the self-curse is forgotten)
Peak Usage (1611)
KJV Hebrews 6:17 -- 'God, willing more abundantly to shew unto the heirs of promise the immutability of his counsel, confirmed it by an oath'
Died still used but trivialized (~1800)
Hebrew shevuah ('sacred self-curse/binding commitment invoking God as witness and enforcer') trivialized into 'saying you promise really hard.' The self-curse dimension -- invoking destruction upon yourself if you break faith -- was lost.
What Replaced It
“promise”
Breakable and common; an oath was an unbreakable, self-cursing commitment
“vow”
Closer but lacks the self-curse: an oath invoked God to destroy you if you lied
“swearing”
Reduced to profanity; swearing was originally the most sacred speech act possible