Exegesis:

This verse is a free quotation from Isa. 66.24.

ho skōlēx autōn ou teleuta ‘their worm does not die’: in the context of Isaiah 66.24, ‘their’ refers to the men who rebelled against God. ‘Their worm’ means ‘the worm that feeds upon them.’

skōlēx (only here in the N.T.) ‘worm’: the singular ‘the worm’ is used generically for ‘the worms’ – it does not mean a single worm. The reference is to worms which feed upon decaying bodies.

teleutaō (cf. 7.10) ‘die’: a ‘worm that does not die’ is a figure of unending decay.

to pur ou sbennutai ‘the fire (of hell) is not quenched’: a figure of unending suffering and torment.

sbennumi (only here in Mark) ‘quench,’ ‘extinguish,’ ‘put out’ (cf. asbestos v. 43).

Translation:

Many languages distinguish clearly between two types of worms: (1) intestinal parasites and (2) worms which feed on carrion and refuse. By translating literally ‘their’ the meaning of intestinal parasites has often been given, but of course this is entirely wrong. The pronoun (in the possessive form in Greek) is by no means possessive. The rotting corpse certainly does not possess the worms – if anything it is the reverse. Accordingly, one must employ a word which clearly refers to worms feeding on dead flesh and indicate the specific relationship, e.g. ‘where the worms feeding on the bodies’ or ‘worms eating their flesh.’ More often than not a plural rather than a singular for ‘worms’ must be used (for one thing they are seldom seen alone), and ‘their’ implies a plural.

Is not quenched is a negative expression of continuous time, hence translatable in some languages as ‘never put out.’

Quoted with permission from Bratcher, Robert G. and Nida, Eugene A. A Handbook on the Gospel of Mark. (UBS Handbook Series). New York: UBS, 1961. For this and other handbooks for translators see here .