My knees are weak through fasting,..... Either voluntary or forced, through want of food or refreshment; this was verified in Christ, when he kneeled and prayed, and his sweat was as it were great drops of blood falling to the ground; see Psalms 69:10.
And my flesh faileth of fatness; or "for want of oil" k; the radical moisture of his flesh being dried up like a potsherd, Psalms 22:15.
k משמן δι' ελαιον, Sept. "propter oleum", V. L. "propter defectum olei", Eth. Arab.