Dual of mar feminine; double bitterness; Merathajim, an epithet of Babylon -- Merathaim.
see HEBREW mar
H4850. Merathayim
מְרָתַ֫יִם used as proper name, feminine = Babylon, מ ׳הָאָרֶץ עַלֿ עָלֶיהָ עֲלֵה Jeremiah 50:21; perhaps = Double rebellion (dual) √ מרה q. v.; DePar 182 thinks = Babylonian Marrâtim, i.e., land by the nar Marrâtu, the bitter river (Persian Gulf) = Southern Babylonia.