Archaic KJV Word
Fast
Modern equivalent: firm
What Was Lost
The link between spiritual fasting and spiritual fastness. To fast (abstain from food) was to make oneself fast (firm/fixed) in devotion. 'Hold fast' meant grip with unbreakable firmness, not grab quickly. 'Stand fast in the faith' meant be immovable, not be speedy. The speed meaning severed fasting from fastness.
Closest Survivor in Modern English
fasten (to make fast/fixed -- the verb preserves the original firmly-attached meaning)
Peak Usage (1611)
KJV Ruth 2:8 -- 'Abide here fast by my maidens'; Judges 16:11 -- 'If they bind me fast with new ropes'
Died ~1800 (the 'firm' sense faded as 'quick' took over)
The older meaning 'firm/fixed/secure/steadfast' (Old English faest) was eclipsed by the speed meaning. Modern readers read 'hold fast' as 'hold quickly' rather than 'hold firmly.'
What Replaced It
“firm”
Merely physical resistance; fast-as-firm meant unshakeable, loyal, permanently fixed
“secure”
Implies safety from threat; fast meant intrinsically immovable regardless of threat
“steadfast”
Closest match but a compound; fast alone once carried the full weight of immovability