John Gill Commentary Isaiah 36:6

"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 to all that trust on him." — Isaiah 36:6 (ASV)

Lo, you trust in the staff of this broken reed, on Egypt , &c.] His ally and auxiliary; and which is rightly called "the staff of a broken reed", if trusted to, and leaned upon, being weak and frail, and an insufficient ground of confidence to depend upon;

the allusion seems to be to the cane or reed which grew upon the banks of the river Nile, in Egypt: whereon if a man lean, it will go into his hand, and pierce it ;

the splinters of the broken reed being leaned on, will enter into a man's hand, and do him harm, instead of being a help to him to walk with: so is Pharaoh king of Egypt to all that trust in him ; pernicious and harmful, instead of being useful and helpful.