The Greek adjective asbestos (ἄσβεστος) means "unquenchable, inextinguishable" — from the alpha-privative a- (not) and sbennumi (to extinguish, quench). In the New Testament it describes the fire of judgment that cannot be put out — eternal, unstoppable consequence.
The word asbestos is used exclusively in contexts of final judgment and hell-fire in the New Testament. Jesus warns of Gehenna with its "unquenchable fire" (pur asbestos) in Mark 9:43–48, quoting Isaiah 66:24. John the Baptist also uses it: the Messiah will baptize with fire that separates the chaff and burns it with "unquenchable fire" (Matthew 3:12; Luke 3:17). The word communicates that the consequences of rejecting God are not temporary but permanent — not reformatory but definitive. This is one of Scripture's most solemn warnings. It is also, in its own way, a mercy: the very severity of the warning is God's urgent call to repentance.