G0762
ἄσβεστος
unquenchable
ἄ-σβεστος, -ον (σβέννυμι), [in LXX for נָפַח לֹא, Job.20:26 א3 A (ἄκαυστον, א2 B)* ;] unquenched, unquenchable: πῦρ, Mat.3:12, Mrk.9:43, Luk.3:17.† (AS)
Full Thayer's lexicon entry
ἄσβεστος — ásbestos
ἄσβεστος, ἄσβεστον (σβέννυμι), unquenched (Ovid,inexstinctus), unquenchable (Vulg.inexstinguibilis): πῦρ, Mat 3:12; Luk 3:17; Mar 9:43, and R G L brackets in 45. (Often in Homer; πῦρ ἄσβεστος of the perpetual fire of Vesta, Dionysius Halicarnassus, Antiquities 1, 76; (of the fire on the altar, Philo de ebriet. § 34 (Mang. i. 378); de vict. off. § 5 (Mang. 2:254); of the fire of the magi, Strabo 15 (3) 15; see also Plutarch, symp. 50:7, probl. 4; Aelian nat. an. 5, 3; cf. Heinichen on Eusebius, h. e. 6, 41, 15).)
4 occurrences
Matthew (1), Mark (2), Luke (1)