The running gag in Incorruptible is that Jack, the layman, always misidentifies the source of a Bible quotation—he mistakes St. Paul for the Pentateuch, that sort of thing. The joke culminates with a particularly venomous curse from Agatha, drawn from Psalms 58:6-8. As Hollinger has it:
AGATHA. …”O God, break the teeth in their mouths…. Like grass let them be trodden down and wither. Let them be like the snail which dissolves into slime, like the untimely birth that never sees the sun!”
Jack guesses Leviticus.
Hollinger helps us out a lot here. Most of the translations of verse 7 employ an image of blunted arrows, rather than that of reaped or withered grass. The King James version of this passage, for instance, is more roundabout:
Break their teeth, O God, in their mouth: break out the great teeth of the young lions, O LORD. Let them melt away as waters which run continually: when he bendeth his bow to shoot his arrows, let them be as cut in pieces. As a snail which melteth, let every one of them pass away: like the untimely birth of a woman, that they may not see the sun.
I found this commentary on Ps. 58 particularly helpful. That image of the dissolving snail is rather fine.