Strong’s Greek
G4643 — sklerotes
The entry below is Strong’s own definition (1890, public domain), and every verse where the word occurs.
σκληρότης
sklerotes — pronounced sklay-rot'-ace
from 4642; callousness, i.e. (figuratively) stubbornness:--hardness. see GREEK for 4642
1 occurrence in the KJV.
Romans 2:5
But after thy hardness and impenitent heart treasurest up unto thyself wrath against the day of wrath and revelation of the righteous judgment of God;