What is the meaning of "Tzuri"?
Rashi: This is like Sal'i (my Rock). A rock is a shelter for travelers, from rain and wind. Radak - here we must say that it is a noun, for Elokei is Samuch to it. In Tehilim (18:3) it says "Keli Tzuri"; both are adjectives. Alternatively, also there Tzuri is a noun (my strength). This is like "Tzur Olamim" (Yeshayah 26:4) - the strength of the worlds and what sustains them.
Malbim (Tehilim 18:3): When Tzur is attributed to a person, it shows special Hashgachah and clinging, which helps to defeat the enemy. This requires three things - Magini (a shield), Keren Yish'i (to gore the enemy) and strength at the time of battle - Misgabi.
What is the meaning of "Echseh"?
Rashi: I will be covered.
What is "Keren Yish'i"?
Radak: Verses refer to strength as Keren, e.g. "v'Karnei Re'em Karnav" (Devarim 33:17). Because the strength of a ram, bull and Re'em 1 can be for harm, and this does not apply to Hashem 2 it says Yish'i - He saves from evil.
Malbim (Tehilim 18:3): It is a horn to gore the enemy.
What is "Misgabi"?
Rashi: He supports me.
Malbim (Tehilim 18:3): He gives to me strength at the time of battle.
What is "Menusi"?
Rashi: I flee to Him for help.
What is "me'Chamas"?
Radak citing Targum Yonasan: It is from extortionists.
Why are there so many metaphors for Hashem's protection and salvation?
Radak (Tehilim 18, citing Shochar Tov): Ten Giborim, haters of David, fell in front of him - Sha'ul, Do'eg, Achitofel, Sheva ben Bichri, Galyus' three brothers and Galyus, and the Mitzri that Benayahu ben Yehoyada killed (23:21). Therefore, David praised Hashem in 10 expressions - Chizki, Sal'i, Metzudasi, Mefalti, Keli, Tzuri, Machsi, Magini, Keren Yish'i and Misgabi. 1
These are the expressions in Tehilim 18; there are some changes from the version here.
