Heb. magen, literally, “shield.” The psalmist’s defense is his reliance upon God as vindicator of the innocent.
In Ugaritic (see pp. 618, 619) the root mgn is used in the sense of “to implore,” or “to beseech.” Hence maginni, “my defence,” should probably be translated “my entreaty,” making the clause read, “my entreaty is unto God.”