C. F. Keil and Franz Delitzsch on 2 Kings 18:21

2Ki 18:21

On Egypt? “that broken reed, which runs into the hand of any one who would lean upon it (thinking it whole), and pierces it through.” This figure, which is repeated in Eze 29:6-7, is so far suitably chosen, that the Nile, representing Egypt, is rich in reeds. What Rabshakeh says of Egypt here, Isaiah had already earnestly impressed upon his people (Isa 30:3-5), to warn them against trusting in the support of Egypt, from which one party in the nation expected help against Assyria.