Look now, you are depending on Egypt, that splintered reed of a staff, which pierces a man's hand and wounds him if he leans on it! Such is Pharaoh king of Egypt to all who depend on him.Sennacherib used this wonderful word picture to communicate a very real danger to King Hezekiah.
The staff had a two-fold purpose for the shepherd: to lend support and balance while walking, and to protect the flock against predatory animals. The beauty and brilliance of using this word picture is that it vividly communicates the irony of being injured by that which one depends on for security and stability.
Another irony is that by heeding Sennacherib's advice to not depend on Egypt, Hezekiah would depend on Yahweh, who is both more stabilizing and secure than even an undamaged staff, and who would ultimately be Sennacherib's undoing.