My suggestion (COSM 90-91) is that the phrase ‘wailing and gnashing of teeth’ combines two Old Testament ideas - ‘wailing’ evokes Israel’s suffering under divine punishment (cf. Micah 7:4), whereas typically the wicked gnash their teeth against the righteous (eg. Psalm 37:12). The linguistic association is fairly clear in the Septuagint. If this is correct, then both the wailing and the gnashing of teeth come from those who suffer divine punishment (understood not metaphysically but historically), but have slightly different connotations.