Exodus 15:15

אלופי אדום אילי מואב [THEN WERE PERTURBED] THE CHIEFTAINS OF EDOM, THE MIGHTY MEN OF MOAB — But surely they had no cause to fear anything because they (the Israelites) were not marching against them! But the explanation is, that they were perturbed by annoyance, because they were annoyed and distressed by the glory that Israel had achieved (cf. Yalkut Shimoni on Torah 251:11). נמגו means THEY MELTED — just as, (Psalms 65:11) “Thou makest it soft (תמגגנה) with showers”. They said, “They are marching against us to destroy us and to take possession of our land” (therefore they melted away from fear, and were not merely perturbed in mind) (Mekhilta).

אָ֤ז נִבְהֲלוּ֙ אַלּוּפֵ֣י אֱד֔וֹם        אֵילֵ֣י מוֹאָ֔ב יֹֽאחֲזֵ֖מוֹ רָ֑עַד        נָמֹ֕גוּ כֹּ֖ל יֹשְׁבֵ֥י כְנָֽעַן׃        

Then the chiefs of Edom shall be amazed; the mighty men of Mo᾽av, trembling shall take hold upon them; all the inhabitants of Kena῾an shalt melt away.