Albert Barnes Commentary Jeremiah 51:1

"Thus saith Jehovah: Behold, I will raise up against Babylon, and against them that dwell in Leb-kamai, a destroying wind." — Jeremiah 51:1 (ASV)

In the midst of them that rise up against me - Or, in Leb-kamai, the cipher for Kasdim, i.e., Chaldaea.

This cipher was not necessarily invented by Jeremiah, or used for concealment. It was probably first devised either for political purposes or for trade, and was in time largely employed in the correspondence between the exiles at Babylon and their friends at home.

Thus, words in common use like Sheshach (Jeremiah 25:26) and Leb-kamai, would be known to everybody.