Adam Clarke on Psalms 40:17

But I am poor - עני ani, afflicted, greatly depressed.

And needy - אביון ebyon, a beggar. One utterly destitute, and seeking help.

The Lord thinketh upon me - The words are very emphatic; אדני Adonai, my prop, my support, thinketh, יחשב yachshab, meditateth, upon me. On which he concludes: "Thou art my help and deliverer." Seeing that my miserable state occupies thy heart, it will soon employ thy hand. Thou, who meditatest upon me, wilt deliver me.

Make no tarrying - Seeing thou art disposed to help, and I am in such great necessity, delay not, but come speedily to my assistance. The old Psalter speaks to this effect: "Let us not be so long under distress and misery that we lose our patience, or our love to thee."