Sheshach is a code word used by Jeremiah.

Jeremiah 25:17. Then I took the cup from the LORD’s* hand and made all the nations to drink, to whom the LORD* had sent me: 18. Jerusalem and the cities of Judah and their kings and their princes, to make them a desolation, an astonishment, a hissing, and a curse, as it is this day; 19. Pharaoh king of Egypt and his servants and his princes and all his people; 20. and all the mixed people and all the kings of the land of Uz and all the kings of the land of the Philistines and Ashkelon and Gaza and Ekron and the remnant of Ashdod, 21. Edom and Moab and the children of Ammon 22. and all the kings of Tyre and all the kings of Sidon and the kings of the isle which is beyond the sea, 23. Dedan and Tema and Buz and all that are in the utmost corners 24. and all the kings of Arabia and all the kings of the mixed people that dwell in the desert 25. and all the kings of Zimri and all the kings of Elam and all the kings of the Medes 26. and all the kings of the north, far and near, one with another, and all the kingdoms of the world, which are upon the face of the earth: and the king of Sheshach will drink after them.