Sheshach

Sheshach - (Jer. 25:26), supposed to be equivalent to Babel (Babylon), according to a secret mode of writing among the Jews of unknown antiquity, which consisted in substituting the last letter of the Hebrew alphabet for the first, the last but one for the second, and so on. Thus the letters sh, sh, ch become b, b, l, i.e., Babel (see: Atbash cipher). This is supposed to be confirmed by a reference to Jer. 51:41, where Sheshach and Babylon are in parallel clauses. There seems to be no reason to doubt that Babylon is here intended by this name. (See Streane's Jeremiah, l.c.)

From Easton's Bible Dictionary (1897)