Lexical Definition

‎ἄσβεστος‎ ásbestos (as'-bes-tos) unquenchable ^G:A
from ‎Α‎ (as a negative particle) and a derivative of ‎σβέννυμι‎

not to be quenched, unquenchable.

• not extinguished, i.e. (by implication) perpetual

Abbott-Smith

ἄ-σβεστος, -ον (< σβέννυμι), [in LXX for נפח לֹא, Jb 20:26 א3 A (ἄκαυστον, א2 B)*;]
unquenched, unquenchable: πῦρ, Mt 3:12, Mk 9:43, Lk 3:17.†

Abbott-Smith References

Liddell-Scott-Jones

ἄσβεστος, ον,

also η, ον [Refs 8th c.BC+]:— unquenchable, inextinguishable, φλόξ[Refs 8th c.BC+] [same place]; not quenched, πῦρ ἄ.[Refs 8th c.BC+]; ἄ. πόρος ὠκεανοῦ ocean's ceaseless flow, [Refs 4th c.BC+]; πῦρ, of hell, [NT]

__II as substantive, ἄσβεστος (i.e. τίτανος), ἡ, unslaked lime, [Refs 3rd c.BC+]
__II.2 a mineral or gem, [Refs 1st c.AD+]. ἀσβεστώδης· tofus, [Refs]

_{LSJ Scripture Refs: Mark.9.43}