Charles Ellicott Commentary 2 Kings 18:21

"Now, behold, thou trustest upon the staff of this bruised reed, even upon Egypt; whereon if a man lean, it will go into his hand, and pierce it: so is Pharaoh king of Egypt unto all that trust on him." — 2 Kings 18:21 (ASV)

The staff of this bruised reed. — Cracked or flawed would be better than bruised; because, as is clear from the following words, the idea is that of a reed splitting and piercing the hand that rests upon it . Regarding the Judean expectations from Egypt, compare to Isaiah 20:1-5; Isaiah 30:1–8; Isaiah 31:1–4, passages in which such expectations are denounced as implying want of faith in Jehovah.