What is "Ahah [la'Yom]"?
Rashi: It is an expression of moaning.
Radak: Woe to the day that will come!
What is "uch'Shod [mi'Shakai Yavo]"?
Rashi: Like the looting of plunderers.
Radak: Just like plundering comes suddenly, so this day will come [suddenly] from Hashem.
Radak citing ha'Nagid: "Mi'Shakai" is from strength. The same applies to "k'Kol Mayim Rabim k'Kol Shakai" (Yechezkel 1:24). Our verse discusses a strong plunderer, that there is no way to escape him.
Malbim: [The day will come] like a plunderer from Hashem, that there is no way to escape him. It is as if he comes from Hashem and His Hashgachah. Even though nature does not obligate plunder, it comes because Hashem is Shoded (overrides) nature - who can stand in front of His anger?!
