from
Easton's 1897 Bible Dictionary
Sheshach
(Jer. 25:26), supposed to be equivalent to Babel (Babylon),
according to a secret (cabalistic) 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. 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.)