Lord of Hosts
in Isaias 9:9, as in many other passages of the Bible, designates God as supreme over untold armies of spiritual and other agencies, which He can employ to give effect to His purposes.
The angels, the stars, as well as armies of men are represented in the Bible as subject to Him.
The Septuagint Version sometimes simply translates the expression Lord of Hosts by a word which means the Omnipotent.
In the text referred to, Isaias says that God, Who is Almighty, will bring about the fulfillment of His prophecy concerning Emmanuel.
New Catholic Dictionary