defend. shield. See note on Isaiah 31:5.
My servant. Three are so called in this book: David (here); Israel or Jacob (the nation) (Isaiah 41:8; Isaiah 42:19; Isaiah 43:10; Isaiah 44:1; Isaiah 45:4; Isaiah 48:20; Isaiah 49:3 and whole chapter); and Messiah (Isaiah 42:1; Isaiah 65:8).
David's sake. Note how Hezekiah refers to these words in his "Songs of the Degrees" (Psalms 132:1; Psalms 132:10).