O thou most proud - זדון zadon . Pride in the abstract; proudest of all people.
Albert Barnes
Notes on the Whole Bible
Babylon is here called Pride, just as in Jeremiah 50:21 she was called Double-rebellion.
Matthew Henry
Concise Bible Commentary
The forces are mustered and empowered to destroy Babylon. Let them do what God demands, and they shall bring to pass what he threatens. The pride of men's hearts sets God against them, and ripens them apace for ruin. Babylon's pride must be her ruin; she has been proud against the Holy One of Israel; who can keep those up whom God will throw down?