|Easton's Bible Dictionary|
Double rebellion, probably a symbolical name given to Babylon (Jeremiah 50:21), denoting rebellion exceeding that of other nations.
Int. Standard Bible Encyclopedia
mer-a-tha'-im (merathayim "double rebellion"): A name used for Babylon in Jeremiah 50:21. According to Delitzsch it may be equivalent to the Babylonian Marratun, i.e. land by the nar Marratu, "the bitter river" (Persian Gulf) = Southern Babylonia (OHL, under the word).
Merathaim (1 Occurrence)
Jeremiah 50:21 Go up against the land of Merathaim, even against it, and against the inhabitants of Pekod: kill and utterly destroy after them, says Yahweh, and do according to all that I have commanded you. (WEB KJV JPS ASV BBE WBS YLT NAS NIV)