"Poison in the pot" or "death in the pot," from the Bible, 2 Kings 4. Quoted in The Commodore by P. O'Brian, p. 279:
"Turning to the priest he cried 'Mors in olla, vir Dei: mors in olla.' ... reverting to Irish he said 'There was the Prophet Eliseus ... and he and his disciples were offered a meal in the desert: but someone cried out in a great voice ... "Do not touch it, oh man of God. There is poison in the pot."'"