"How is Sheshach taken? and how is the praise of the whole earth surprised? how is Babylon become an astonishment among the nations?" (Jeremiah 51:41 KJV)
Barry J. Beitzel writes:
"Atbash. Atbash is an oratorical device according to which letters of one or more words, counted from the beginning of the alphabet, are exchanged for corresponding letters counted from the end of the alphabet (e.g.' = t, b = s, etc.). Embedded in Jeremiah's grim oracle of doom directed against Babylon and the king of Babylon (chaps. 50-1) is the enigmatic Sheshak (51:41). Enigmatic, that is, until one recognizes that the letters which comprise the word ssk are actually atbash for bbl, 'Babylon' (cf. 25:26). In this same chapter (v 1), Jeremiah describes the inhabitants of Babylon by means of the otherwise mysterious lb qmywhich, through atbash, becomes ks'dym, 'Chaldeans,' known to have been contemporary inhabitants of the great city."
Atbash is a form of paronomasia. See Beitzel, "Exodus 3:14 and the Divine Name: A Case of Biblical Paronomasia." Trinity Journal 1 NS (1980): 5-20.
We read the following in John Calvin's commentary on Jeremiah and Lamentations:
"But he calls Babylon here Sheshach, as in Jeremiah 25. Some think it to be there the proper name of a man, and others regard it as the name of a celebrated city in Chaldea. But we see that what they assert is groundless; for this passage puts an end to all controversy, for in the first clause he mentions Sheshach, and in the second, Babylon. That passage also in Jeremiah 25 cannot refer to anything else except to Babylon; for the Prophet said, 'Drink shall all nations of God's cup of fury, and after them the king of Sheshach,' that is, when God has chastised all nations, at length the king of Babylon shall have his turn. But in this place the Prophet clearly shows that Sheshach can be nothing else than Babylon. The name is indeed formed by inverting the alphabet."