Albert Barnes Commentary Job 4:3

"Behold, thou hast instructed many, And thou hast strengthened the weak hands." — Job 4:3 (ASV)

Behold, thou hast instructed many - That is, you have instructed many how they ought to bear trials, and have delivered important maxims to them on the great subject of the divine government. This is not designed to be irony, or to wound the feelings of Job. It is intended to recall to his mind the lessons which he had inculcated on others in times of calamity, and to show him how important it was now that he should reduce his own lessons to practice, and show their power in sustaining himself.

Thou hast strengthened the weak hands - That is, you have aided the feeble. The hands are the instruments by which we accomplish anything, and when they are weak, it is an indication of helplessness.